Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea

Transcription

1 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number pp Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Zhenglong Jiang 1*, Junzhan Zhu 2, Shiping Wei 1, Baolin Liu 1 1 School of Marine Sciences, China University of Geosciences, 29 Xueyuan Rd. Handian District, Beijing , China 2 Research Institute, Shenzhen Branch Company, CNOOC Ltd., 68 Jiangnan West Rd. Haizhu District, Guangzhou , China *Corresponding author jiangzl@cugb.edu.cn (Received 29 May 2012; accepted 28 September 2013) Abstract Huizhou sag is located in the depression zone in the north of the Pearl River Mouth basin. In recent years, great progress has been made in the exploration focused on the deep Paleogene, opening up new areas of exploration and necessitating the evaluation of both the hydrocarbon generation potential and the exploration potential of the Paleogene. The Enping Formation and the Wenchang Formation have become new exploration series of strata. The research utilizes paleotectonic recovery, sequence stratigraphy analysis, basic parameter prediction of hydrocarbon source rock and basin modeling, as well as other techniques. It is carried out to examine the tectonic inversion effect of the Huidong low uplift, the sedimentary facies of the Wenchang Formation, hydrocarbon source rock thickness and organic carbon content in the series of strata that have not been extensively drilled, with the calculation of both the hydrocarbon generation intensity and amount from the source rock. It is considered that: (1) after sedimentation, the Wenchang Formation suffers from tectonic inversion and severe uplift and denudation, especially the Huidong low uplift, changing the sedimentary pattern; (2) during the sedimentation of the Wenchang Formation, LF13 low-lying to Huizhou 14 low-lying is at the center of the lake basin, dominated by deep lake - semi-deep lake facies, with hydrocarbon source rock developing; (3) in total, t of hydrocarbon is generated in Huizhou sag, with total oil and gas resources with the Wenchang Formation as source rock at about t; (4) comparing with the parameters and results of the previous resources assessment and considering the exploration practices in the area, it is believed that the deep part of Huizhou sag still has great exploration potential as a sag with large amount of remaining oil and gas resources in Zhu I depression. Keywords: Inversion structure, Sedimentary facies, Wenchang Formation, Resource potential

2 390 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea 1. INTRODUCTION The Zhu I depression is an important oil production base in China s coastal areas (Long et al., 2006; Li et al., 1997). The Tertiary system in the depression has two layers (Li, 1993; Wang et al., 2008). The Neogene of the upper structure features a depression structure and is dominated by marine strata, whereas the Paleogene of the lower structure features a rift structure and is dominated by continental deposits (Chen et al., 2003; Gong, 2004). In the 1990s, oil and gas exploration was mainly concentrated in the marine strata, where over 600 million tons of reserves have been discovered and more than 10 oilfields have been developed. In recent years, oil and gas exploration in the deep depression has indicated that the Paleogene of the lower structure also has the potential for oil and gas exploration. Today, a reservoir has been found as good oil and gas shows detected in the drilling of Huizhou sag (Nie et al., 2011), such as HZ9-2, HZ19-1, HZ19-2 and HZ19-3 wells. As one of the main series of strata in the Pearl River Mouth basin with potentially valuable resources, the Paleogene shall become an exploration area to replace the dwindling eastern reserves of the Pearl River Mouth basin. As few exploration wells are encountered, in addition to obtaining a small amount of well test data for the basic parameters of the Wenchang Formation hydrocarbon source rock, it is necessary to establish the connection between the sedimentary facies and the hydrocarbon source rock thickness, organic matter abundance, and the hydrocarbon production rate. This will reveal the plane distribution of the basic parameters of hydrocarbon source rock. In the research it is found that after sedimentation the Wenchang Formation has strong denudation, as well as tectonic inversion and changes in the tectonic pattern. This suggests the new idea that the Wenchang Formation of Huidong low uplift is deeper lake - deep lake deposits, greatly improving the understanding on hydrocarbon generation conditions of the Wenchang Formation and laying foundations for resources reevaluation. With the extent of oil and gas exploration in the Zhu I depression generally broadening, the quality of seismic data has improved, and sequence stratigraphy et al., 2007; Fu et al., 2001Zhu I depression. Thoroughly analyzing the distribution of effective hydrocarbon source rock and oil and gas resources can provide a sound basis for decision-making with respect to oil and gas exploration. 2. GEOLOGICAL BACKGROUND Huizhou sag is located in the Pearl River Mouth basin, which is located in the north of the South China Sea on the southern edge of China (Fig. 1). The basin is oriented SW-NE, parallel to the South China continental shelf and slope. The Pearl River Mouth basin includes five regional tectonic units: northern uplift, northern depression, central uplift, southern depression and southern uplift (Chen et al., 2003). Huizhou sag is located in the center of the northern depression zone, composed of five half-grabens and two low uplifts (Fig. 1).The Cenozoic group of Huizhou sag is divided into a total of 8 formations, listed from the oldest to the most recent: the Paleogene Shenhu Formation, the Eocene Wenchang Formation, the Eocene-Oligocene Enping Formation, the Oligocene Zhuhai Formation, the Neogene Miocene Zhujiang Formation, the Miocene Hanjiang Formation and Yuehai Formation, and the Pliocene Wanshan Formation (Fig. 2).

3 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 1. Tectonic units map of Huizhou sag. The Huizhou sag is located in the center of the northern depression zone in the Pearl River Mouth basin, composed of five half-graben and two low uplifts, which are Huixi half-graben, Huibei half-graben, Huinan half-graben, Huidong half-graben, Luxi half-graben, Huizhong low arch and Huidong low arch. Figure 2. Cenozoic histogram of Huizhou sag. The Cenozoic group of Huizhou sag is divided into a total of 8 formations, listed from the oldest to the most recent: the Paleogene Shenhu formation, the Eocene Wenchang formation, the Eocene- ligocene Enping formation, the Oligocene Zhuhai formation, the Neogene Miocene Zhujiang formation, the Miocene Hanjiang formation and Yuehai formation, and the Pliocene Wanshan formation. The Wenchang formation and Enping formation are dominated by the Lake facies and limnetic facies, and high quality source rock and cap rock are developed with some sandstone reservoirs. Zhuhai Formation develops sandstone reservoir with local mud cap rock. The Wenchang Formation and Enping formation can be an effective combination of the source rock, reservoir and cap rock.

4 392 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea The Wenchang Formation is dominated by grey black mudstone, interbedded with a small amount of gray sandstone. The gray sandstone is interbedded with coal beds in the upper part of some areas, representing lake deposits. The Enping Formation is dominated by a set of alternating layers of gray black mudstone and sandstone interbedded with coal beds, and represents a set of paludal facies deposits. From bottom to top, three positive cycles are formed. The Wenchang Formation is the main source rock of this depression. 3. RESEARCH IDEAS AND METHODS 3.1. Research ideas For the Huizhou sag lacking numbers of significant exploration wells and having the Wenchang Formation as the main hydrocarbon source rock, accurate estimation of the amount of oil and gas resources requires the acquisition of the basic parameters of the Wenchang Formation hydrocarbon source rock, such as hydrocarbon source rock thickness, organic matter abundance, hydrocarbon production rate, etc. Generally, there is a certain correlation between the dark mudstone content in lake deposits, organic matter abundance and the sedimentary facies. Generally, from the deep lake facies, semi-deep lake facies, shore-shallow lake facies to the delta facies, both the hydrocarbon source rock content in dark mudstone and the organic matter abundance decrease gradually. Based on such trends, the relationships and the trends between the basic parameters of the hydrocarbon source rock and sedimentary facies can be established by examining hydrocarbon source rock content and the measuring geochemical data of drilled wells. In this way, the profile of the basic parameters of the hydrocarbon source rock can be prepared (Fig. 3). Geochemical analysis Sedimentary facies Hydrocarbon source rock thermocompression simulation experiment Sequence thickness Organic matter abundance Hydrocarbon source rock content Plane distribution of organic matter abundance Source rock distribution Hydrocarbon generating plate Hydrocarbon generation simulation Hydrocarbon generation amount calculation Figure 3. Research ideas for hydrocarbon source rock and hydrocarbon generation amount calculation

5 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number By means of basin modeling, hydrocarbon generation is simulated, and the hydrocarbon generation amount in the Wenchang Formation is calculated in different geological periods. By referring to the hydrocarbon migration and accumulation coefficients of different basins in previous nationwide oil and gas resource evaluations, prospective resources with the Wenchang Formation as source rock are estimated, providing reference for the further exploration of the Paleogene in Huizhou sag Calculation method for hydrocarbon generation quantity For each computing element and every set of hydrocarbon source rock, the hydrocarbon generation amount is calculated according to the following formula: Q p = S H ρ r C% R p Where: S Source rock area, km 2 ; H Source rock thickness, m; ρ r Source rock density,g/cm 3 ; C% Average parent material abundance of source rock in organic carbon content, %; R p Average oil and gas amount generated per ton of organic carbon in the geohistory, kg/t c ; Q p Total amount of oil and gas generated by source rock. The oil is in 10 6 tons units. The hydrocarbon generation amount divided by the area gives the hydrocarbon generation intensity (E, in 10 6 tons/km 2 ), which is calculated according to the following formula: E = (S H ρ r C% Rp)/S=H ρ r C% R p 3.3. Resource amount calculation method As a traditional and primitive method for oil and gas resources evaluation, the hydrocarbon generation rate is widely applied. Its essence is to calculate the hydrocarbon generation rate of the source rock, and then calculate the amount of resources based on comparing the volume of source rock and oil and gas accumulation to discharge coefficient in the study area, with the formula as follows: Q=Q p K ma Where: Q p Total amount of oil and gas generated by source rock. The oil is in 10 6 ton units; K ma Oil and gas accumulation to discharge coefficient refers to the ratio between the amount of oil and gas accumulated into reservoir and that generated from source rock accumulation; Q Amount of oil and gas resources, 10 6 ton units.

6 394 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea 4. INVERSION STRUCTURE AND SEDIMENTARY FACIES OF THE WENCHANG FORMATION 4.1. Identification of inversion structure in Huidong Low Uplift Huizhou sag is composed of five half-grabens and two low uplifts (Fig. 4). The sag is a typical rift basin. The Cenozoic tectonic events of the Pearl River Mouth basin is mainly shown in the unconformity on the seismic profiles, sediment or biological discontinuity, rupture and magmatic action, etc. There are seven seismic reflection unconformities, three of which are regional, namely, Tg, T8 and T7. On the seismic profiles, stratigraphic overlap is very common on the top of the Wenchang Formation. The unconformity between the Enping Formation and the Wenchang Formation is caused by an important tectonic event. There are five important Cenozoic tectonic movements: Shenhu Movement, the first scene of Zhuqiong Movement, the second scene of Zhuqiong Movement, Nanhai Movement and Dongsha Movement (Li, 1993; Luo, 1997; Wang, 1998). At the end of the Wenchang Formation forming period, the sedimentary basin experienced uplift denudation after extensional fracture development, and had intense tectonic inversion (Jiang et al., 2008). Figure 4. Seismic profile location and major oilfields in the Huizhou sag Previous researchers focused more on the relation between the inversion structure and oil-gas reservoir forming (Chen et al., 2003; Fu et al., 2001; Jiang et al., 2008; Li, 1993; Li and Lin, 2006; Luo, 1997; Zhang et al., 1996; Zhao et al., 2007; Zheng and Fu, 1997; Zhu et al., 2007) and less on the control of the inversion structure on the sedimentary environment and resource potential. The inversion structure in the faulting

7 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number period changes the deposit pattern and the identification of the depocenters, whereas the reconstruction of depositional settings have direct influence on the evaluation of resource potential and the deployment of exploration. Therefore, the research on the sedimentary environment and the recovery of inversion is of great significance. The second scene of Zhuqiong Movement occurred between the middle and late Eocene and Early Oligocene, 39 ~ 36 Ma ago. The movement caused the Pearl River Mouth basin to again experience uplift and denudation associated with fracture and magmatic action. This movement s duration was the longest and caused the strongest uplift and denudation, with limited remnants, so that the entire Upper Eocene was missing. It was the most important tectonic movement at the faulting stage of the basin. It is reflected in the regional unconformity on the seismic profilest 8 Enping Formation. An erosional surface was formed in the uplift area. The Enping Formation was generally missing, and the Zhuhai Formation directly covered the bedrock. The Huidong low uplift was a center of subsidence during the sedimentation of the Wenchang Formation, experiencing tectonic inversion after uplift and erosion after the sedimentation of the Wenchang Formation (the second scene of Zhuqiong Movement). Then, the Wenchang Formation suffered from severe denudation and formed low uplift. The identification of positive inversion structure can be ascertained from the perspective of seismic reflection characteristics, sedimentary facies and the incoordination of tectonic position (Yang et al., 2006; Shang et al., 2001; Li, 1996) Seismic reflection characteristics of the Wenchang Formation Consider Huidong low uplift as an example to study seismic reflection characteristics of the Wenchang Formation by 95EC-HZ1 profile. The northeast trending profile (Fig. 5) is located in the east of Huizhou sag and passes HZ24 low-lying, LF13 low-lying, Huidong low uplift and LF7 low-lying from south to north. Seismic reflection of Wenchang Formation in LF13 low-lying is basically parallel to the base, and the truncation phenomenon below the top surface (T8 reflection surface) is obvious (Fig. 5), indicating that LF13 low-lying Wenchang Formation suffers from late tectonic inversion, with strong uplift and denudation.hz24 low-lying Wenchang Formation and LF13 low-lying have similar features.in this view, at the time of Wenchang Formation, the area of LF13 low-lying and HZ24 low-lying was a connected lake basin Semi-deep to deep lake facies distributed on Huidong low uplift LF Well is located in the upper part of Huidong low uplift, and is the only drilled well in Huizhou sag revealing the semi-deep lake strata in the Wenchang Formation, which is 92.4m thick with mudstone accounting for 92.6%. Analyzed from the lithology and electric curve, lake transgressive cycle characteristics are significant. Furthermore, from a paleontological point of view, Wenchang Formation contains only organic debris combinations dominated by amorphous organic matter, and the sedimentary environment is a semi-deep lake with high productivity and dominated by freshwater.

8 396 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea S SB6 SQ6 SQ5 SB6 SQ6 SQ2 SQ5 SQ4 SQ2 SQ5 SQ6 SQ5 SQ2 T6 T7 SB6 T8 Tg LF7 lowlying LF13 lowlying HZ24 lowlying Figure 5. The 95EC-HZ1 seismic reflection and interpretation profile. The seismic reflection interfaces of the T6, T7 and T8 are shown in Figure 2. The Tg is the bottom reflection of the Cenozoic. The SQ 2 is a sequence in the Wenchang formation, and the SQ 5 and SQ 6 are the sequences of the Enping formation. The SB6 is the sequence boundary between SQ 5 and SQ 6. The Enping formation is the thickest in the LF13 Low-lying and the HZ24 Low-lying with three sequences developed, reflecting theactivities of faults in the southern boundary were enhanced. Obviously controlled by the faults and palaeogeomorphology, the Wenchang formation in the center and north of the sag was thicker than the south, which is thickest in the LF13 low-lying with the SQ 2 and SQ 3 generally denuded. According to seismic reflection characteristics of Wenchang Formation from LF13 low-lying to Huidong low uplift, LF Well sedimentary facies and analysis of semi-deep lake mudstone as oil sources of this area (Fig. 6), the low uplift where LF Well is located should be deep sag of the lake basin spanning the period from during the sedimentation of Wenchang Formation to after the denudation at the end of Wenchang Formation sedimentation before Enping Formation sedimentation, specifically tectonic inversion denudation by the second scene of Zhuqiong Movement. The inversion changes the pattern of the lake basin. As forhuidong low uplift, recovery of the sedimentary evolution history can help determine the range of the lake basin (Woodward et al., 1985; Athy, 1930; Dahlstrom, 1969; Cham berlain, 1910; 1919; Gidds, 1983).

9 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 6. Sequence stratigraphy analysis of LF well. SQ 2 sequence of the wenchang formation in the well LF is mainly mudstone. Sandrock, siltstone and mudstone develop as the shore-shallow lake facies at the bottom. The clean mudstone develops as the semideep lake facies at the top Semi-deep lake oil sources The ratio of C 30 4-methyl steranes/c 29 regular steranes (M/Z217) of Wenchang Formation in this area generally exceeds 0.2, which exceeds 1.0 in deep lake mudstone. The hydrocarbon source rock in different facies zones can be distinguished by a graph of the relationship between bicadinanes/c 30 hopane and C 30 4-methyl steranes. C 19 /C 23 tricyclic terpanes (M/Z191) ratios of less than 0.5 can be used as the fingerprint information for deep lake source rock of the Wenchang Formation and C 19 /C 23 tricyclic terpanes (M/Z191) ratios of less than 1.0 can be used as the fingerprint information for shore-shallow lake source rock of Wenchang Formation (Fig. 7). In the graph of relationship between bicadinanes/c 30 hopane and C 30 4-methyl steranes/c 29 sterane (Fig. 7), it can be deduced that the oil of HZ18-1-1, LF and LF Wells comes from deep lake hydrocarbon source rock of the Wenchang Formation. The oil of Huidong low uplift comes from semi-deep lake hydrocarbon source rock of the Wenchang Formation (Zhu et al., 2007; Fu et al., 2001), indirectly revealing that the lake basin of the Wenchang Formation is big in range and that tectonic inversion occurred.

10 398 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Figure 7. Crossing of bicadinanes/c 30 hopane and C methyl steranes. The good correlation between crude oil in the HZ18-1-1, LF13-1 and LF13-2 wells and the biomarkers from the source rocks of deep lake facies in the Wenchang formation shows that the source rocks are the origins of the crude oil.

11 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 8. Stratigraphic subdivision program for Wenchang formation sequence. Two sequence boundaries of the SB 2 and SB 3 within the Wenchang formation were developed, and seismic reflections have the on lap, top-lap or truncation appearance. The Wenchang formation is divided into three sequences of the SQ 1, SQ 2 and SQ 3 ; the Enping formation also can be divided into three sequences of the SQ 4, SQ 5 and SQ Wenchang Formation sedimentary facies According to sequence stratigraphy (Deng, 1995; Deng and Cross, 2002; Deng et al., 2000), 3 third-order sequences (SQ 1,SQ 2,SQ 3 ) (Deng et al., 2008 ) can be identified within the Wenchang Formation, respectively corresponding to the initial rift filling stage, the rift expansion stage and the rift shrinking stage. The sequence boundaries mainly feature local unconformities or toplaps. The SQ 2 sequence is distributed in the Huizhou sag, whereas the SQ 1 sequence belongs to the early filling stage and is distributed in the local part of the Huizhou sag. Huibei and Huixi half-grabens have SQ 3 sequence residues, and the Wenchang Formation in the Huidong area is mainly SQ 2 sequence. Therefore, study of the Wenchang Formation takes the SQ 2 sequence as representative. According to the recovery of inversion structure combined with other drilling and seismic data analysis, it is considered that this area was an entire lake basin during the sedimentation of SQ 2, with main provenance mainly distributed in the northwest and northeast, followed by southern provenance.huidong low uplift is a denuded zone after the sedimentation of Wenchang Formation and obviously does not separate Huinan half-graben from Lufeng half-graben. In the sag, there are only 5 drilled wells revealing Wenchang Formation and they are located in different half-grabens. Well-to-seismic calibration and constitution analysis of seismic sequence stratigraphy show that Wenchang Formation revealed by drilled wells is mainly strata of SQ 2 sequence. SQ 2 sequence revealed by half-grabens

12 400 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea shows a clear asymmetry, forming a positive lake transgressive cycledue to denudation, the strata stored during the deposition of SQ 2 sequence Maximum flooding surface of Wenchang Formation appears during the development of the SQ 2 sequence. According to low-lying structure-sedimentary evolution history analysis and indications from well-to-seismic calibration, the Wenchang Formation revealed by drilled/logged wells of different half-grabens is entirely in SQ 2 sequence. The sequence in each well shows asymmetric cycles are mainly under the action of sedimentation during datum plane rise, indicating that itbase subsidence rate and accommodation space gradually increased. The upper part of the sequence generally develops a thick dark gray mudstone member with pure lithology or mudstone member interbedded with thin layers of siltstone. The maximum residue thickness of dark gray mudstone can reach about m, such as in the HZ Well and LF Well, which were deep lake deposits at the time of the maximum flooding. Seismic reflection characteristics also show that the secondorder sequence has obvious overlap characteristics, with the maximum lake facies exceeding those in the current SQ 2 period other than those in the SQ 3 sedimentation period, especially in the Huixi and Huinan half-grabens, where the SQ 2 shows obvious overlap characteristics. The maximum flooding surface in second-order sequence of the Wenchang Formation is within the SQ 2 sequence. During the sedimentation of SQ 2, Huizhou sag experiences strong faulting with deep lake basins and develops a fluvial delta-shore-shallow lake facies-semi-deep lake deposit system (Deng et al., 2008 ) (Fig. 9). The fluvial delta deposit system is mainly Figure 9. Huizhou sag Wenchang formation SQ 2 sedimentary facies. During the sedimentary period of the Wenchang formation, the sag developed. The semideep lake facies was distributed in the sag near the boundary faults, and the facies in the Huixi half-graben, Huidong half-graben and Huibei half-graben was well developed. By the research on the palaeostructure reconstruction, sedimentary facies and oil/gas sources, it s approved that the Huidong low arch was developed during the sedimentary period of the Wenchang Formation, when the arch developed the semideep lake facies. The half-grabens are located as Figure 1.

13 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Wenchang Figure 10. Semi-deep lake seismic reflection characteristics. Log facies is same as in Figure 6. semideep lake facies The lake center appears the middle and high continuous, middle amplitude sheet facies or weak amplitude white facies, represent for pediocratic environment. With the background of low-frequency weak reflection, interrupted continuous middle amplitude reflection seismic facies was viewed with the lithology mudstone. developed at the western edge of the sag The transition zone has obvious control on the provenance, for example, the transition zone along the south develops braided deltas and fan deltas, while the provenance at the scarp of the fault directly goes into the lake and forms nearshore subaqueous fans. On a seismic profile, semi-deep lake strata show parallel, continuous, low-frequency and strong reflection (Fig. 10). 3 major semi-deep lake areas are identified based on seismic reflection characteristics, namely, XJ24-XJ30-HZ26 low-lying lake basin center, LF13-HZ14 low-lying lake basin center and HZ8 HZ10 low-lying lake basin center. 5. PREDICTION OF HYDROCARBON SOURCE ROCK IN SERIES OF STRATA WITH LITTLE DRILLED WELLS According to the recovery of inversion structure, the sedimentary facies study, correlations in strata between strata thickness and hydrocarbon source rock content in strata, as well as results from organic matter abundance analysis, the contour charts can be prepared for the thickness and organic matter abundance of hydrocarbon source rock Thickness of hydrocarbon source rock According to statistics of drilling lithology in the LF Well Wenchang Formation, the hydrocarbon source rock content is greatest in semi-deep lake facies, reaching 82.5%, followed by that in shallow lake facies (Table 1). This time, we set the boundary of hydrocarbon source rock content in semi-deep and shallow lake facies at 70%, that in shallow lake facies between 70%-50% and that in other sedimentary

14 402 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Table 1. Statistics of mud hydrocarbon source rock content in Huizhou sag. Well Depth Facies Source rock (%) LF Mid-deep lake 82.5 XJ fluvial 15.3 XJ24-3-1AX Shallow lake 62.7 HZ Shallow lake 41.1 Notes: Based on the sedimentary facies from the well research, source rock contents of different sedimentary facies were tested according to the drill core and sieve residue log. The content in the semideep lake facies is greatest reaching over 80%, and lowest in the fluvial facies less than 20%. facies at less than 50%. By this criterion, the mud hydrocarbon source rock content chart is prepared for various sequences of Wenchang Formation. When this chart is multiplied by the strata thickness chart, the contour chart for hydrocarbon source rock thickness in various layers can be obtained. From the contour chart, it can be seen that the hydrocarbon source rock of the Wenchang Formation in Huizhou sag is widely distributed and is generally m thick (Fig. 11). The hydrocarbon source rock in the Huixi and Luxi half-grabens is thick and develops several thickness centers. Figure 11. The contour chart for hydrocarbon source rock thickness in Huizhou sag SQ 2 sequence. The source rocks are over 500 m thick in the Huixi half-graben, Huidong half-graben and Huibei half-graben, and distributed widely. The halfgrabens are located as Figure 1.

15 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Organic matter abundance The rock pyrolysis analysis and statistics data of the study area show that the distribution of organic matter abundance in hydrocarbon source rock in both the Wenchang Formation SQ 2 sequence semi-deep lake facies and in that of the Enping Formation sequences follows a certain trend (Shi et al., 2009) (Fig. 12, Table 2). According to the rock pyrolysis experiment data of 4 key wells, the organic carbon content boundary (or standard, benchmark ) of semi-deep and shallow lake facies is set at 2%, and that of shallow lake facies is 1%-2%. From this, the contour chart for organic carbon content in the Wenchang Formation is prepared (Fig. 13, Table 2). The Huixi area and Luxi half-graben sedimentary facies are mainly semi-deep lake facies and have the highest organic carbon content. Figure 12. Distribution of organic matter abundance in Hydrocarbon source rock of different SQ 2 Sequences in Zhu I depression Wenchang formation (Shi Hesheng et al., 2009). Table 2. Organic matter abundance statistics of hydrocarbon source rock in the Huizhou sag. Well Facies Samples TOC (%) S 1 +S 2 (mg/g) Hydrogen index (mg/g.toc) XJ fluvial HZ Shallow lake LF Mid-deep lake XJ Shallow lake Notes: According to the geochemical analysis and the result of the well sedimentary facies, organic matter abundances of different sedimentary facies were counted. The abundance is greatest in the in the semideep lake facies and lowest in the fluvial facies.

16 404 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Figure 13. The contour chart for organic carbon content of Hydrocarbon source rock in Huizhou sag SQ 2 sequence. The Huixi half-graben and Huidong half-graben, with high organic carbon content of the Wenchang formation generally over 2%, are the major source rock distributed area. The oil sources are located in the two sags or periphery. The half-grabens are located as Figure 1 and the oil fields as Figure HYDROCARBON GENERATION AND AMOUNT OF RESOURCES 6.1. Hydrocarbon generation calculation Hydrocarbon source rock of the Kerogen type is measured by laboratory pyrolysis experiments. Kerogen in the hydrocarbon source rock of the Wenchang Formation is mainly II 1 type. Brown mudstone of the Wenchang Formation is deep lake mudstone with high organic carbon content and II 1 type organic matter. Thermo-compression simulation experiments of the LF Well show that R o falls between 0.6~1.2% during the main hydrocarbon generation stage and that the hydrocarbon generation rate can reach 383.6mg/g TOC (Table 3). Basin modeling is performed and a SQ 2 hydrocarbon generation intensity contour chart (Fig. 14) is prepared by using the thermo-compression simulation experiment results, the hydrocarbon source rock thickness and organic matter abundance contour chart, and examining the combination of the overlying strata and thermodynamic parameters (Chen et al., 2003). The total hydrocarbon generation quantity of 5 halfgrabens is t, and that of the Wenchang Formation is t. The Huidong half-graben, covering an area of 1312 km 2, includes LF13 Wenchang Formation sub-sag and LF13 Enping Formation sub-sag, as well as LF13 tilting tectonic belt and HZ18 tilting tectonic belt.the main hydrocarbon source rock of Huidong half-graben is SQ 2, which generates t of oil in total, t of gas in total and t of hydrocarbon in total.

17 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Table 3. Brown mudstone simulation experiment data of the Wenchang formation in LF well. Light Residual Total Temperature Sample TOC Total Hydro-carbon Condensates Total Oil oil hydrocarbon o C (g) (%) gas (ml) (mg) (mg) oil(mg) (mg) (mg) (mg) R o (%) Original sample Notes: Thermocompression simulation experiments were run in the Huabei oilfield. The source rock was divided into five copies and 80 g per copy. Four copies ones were placed in the autoclave with 5MPa nitrogen gas, added water with the weight 15% of the rock sample, and were warmed reaching 200 C, 300 C, 400 C and 500 C, respectively, and then thermostatic control for 24 hours. Figure 14. Current Hydrocarbon generation intensity contour chart of Huizhou sag SQ 2 Hydrocarbon generation intensity of the source rock in the Wenchang formaiton of Huixi half-graben is greatest reaching over tons/km 2, and the favorable hydrocarbon generation area is wide as the major hydrocarbon generation sag. The Huidong and Huibei half-graben is next with the hydrocarbon generation intensity of above tons/km 2. The half-grabens are located as Figure 1.

18 406 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Table 4. All previous hydrocarbon generation amount and resource amount estimates of the Huizhou sag. Year Researcher Method Hydrocarbon Generation Amount Expulsion-accumulation coefficient (%) Oil and gas resource 1986 Tang Zhenyi 1992 Lin Hongzhi Expulsion-accumulation coefficient Chloroform bitumen "A" method Oil Oil Yang Jiaming Basin modeling Oil Chloroform bitumen "A" Oil method Al analogy method Oil Oil Basin modeling Gas Zou Yechu Chloroform bitumen "A" method Oil % Deng Hannan Oil 144 Gas Cai Yongwei Oil Gas Unit: Oil/ 10 8 t, Gas/10 11 m 3, oil and gas resource/10 8 t. Notes: In the situation of different periods and degree of exploration, the previous researchers have estimated the hydrocarbon generation amount of Huizhou sag with major difference based on various data and different geologic concept. Hydrocarbon generation amount is less than t, and oil and gas resource is less than t. Since the 1980s, there have been estimates on the hydrocarbon generation amount of Huizhou sag based on various data, as shown in Table 4. In 1986, Tang et al. (1986) estimated that the series of strata would generate t of hydrocarbon in total; in 1992, Lin et al. (1992) estimated by basin modeling that the total hydrocarbon generation amount was t; in 1993, Yang (1993) estimated by basin modeling that t of oil and m 3 of gas in total were generated; in 1994, Zhou et al. (1994) estimated by the chloroform bitumen A method that the maximum hydrocarbon generation amount was t; in 1996, Zhang Q.Y. et al. (1996) estimated that t of hydrocarbon was generated in total; in 2000, Cai (2000) estimated that t of hydrocarbon was generated in total. Though above estimations of hydrocarbon generation amount are huge, the oil and gas resources are all less than t.

19 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Resource amount calculation The migration and accumulation coefficient is a key parameter for resource quantity calculation by means of the hydrocarbon generation rate and has direct influence on the calculation results.the migration and accumulation coefficient is formulated mainly by consideration of the hydrocarbon kitchen development scale and period, structural development characteristics, order, location, Pathway system characteristics and preservation conditions. In the new resource evaluation, OGRSRC (2010) presented statistics on the oil migration and accumulation coefficient of migration and accumulation units that had undergone a high degree of exploration, have proven resources and about which much is known geologically. Data on these units, such as the Songliao, Bohai Bay, Ordos, Tarim, Junggar, and Turpan-Hami Basins, show that the Cenozoic oil migration and accumulation coefficient is between 9-11% and although it varies in different areas to different extents of evolution, the minimum is greater than 5%. The Langfang Branch of PetroChina made statistical analysis on 214 data of 14 major gas-bearing basins, and divided the reservoir forming conditions into four categories: gas-rich, medium gas-rich, common gas-rich, poor gas-rich, with accumulation coefficient > 15, 6-15, 2-6, <2, respectively. Huizhou sag is Cenozoic sag with rich oil and lean gas for reservoir formation, for which the accumulation coefficient of crude oil can be assumed to be around 10% and natural gas should be under 2. Considering that the oil generated in different periods has different contributions for the reservoir formation, the oil and gas generated earlier and diffused more easily, and the oil and gas reservoirs formed earlier were more susceptible to be damaged, so the accumulation to discharge coefficient of the oil and gas generated in different periods should be different. Along with the improvement of information and the advancement of technologies, oil and gas discoveries in Wenchang and Enping Formations in recent years provide the opportunity for effective evaluation of Huizhou sag resources. Further understanding of resource potential is based on in-depth study of basic parameters and new exploration achievements and new exploration understanding. The value of the Cenozoic oil and gas accumulation coefficient in this area is determined in four stages: during the sedimentation of the Wenchang Formation to the Enping Formation (49-30Ma), during the sedimentation of the Zhuhai Formation-lower part of Zhujiang Formation ( Ma), during the sedimentation of upper part of the Zhujiang Formation to Hanjiang Formation ( Ma), and from the Yuehai Formation to the present. By referring to previous research findings and by considering geological reservoir forming conditions in this area, the oil migration and accumulation coefficient can be assumed to be 13%, 10%, 9% and 7%, respectively, while that of natural gas can be assumed as 12, 10, 8 and 6 respectively (Zhou et al., 2007). Huizhou sag Wenchang Formation is estimated to have oil resources of t and gas resources of t, equivalent to m 3, with the total amount of resources up to t of oil equivalent (Table 5).Previous hydrocarbon generation amount estimations are very different, as shown in Table 4, being less than 50% of this new estimation. This is mainly due to improvements of basic data quality and a new understanding of Huizhou sag structure and sedimentary

20 408 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Table 5. Oil and gas resources estimate of the Wenchang formation in Huizhou sag. Reserve Stage (Ma) expulsion-accumulation coefficient Sub-total Oil reserve SQ SQ SQ total expulsion-accumulation coefficient Natural gas reserve SQ SQ SQ total Unit: oil/10 9 tons; gas/10 9 m 3. Notes: Based on the sedimentary facies, basic parameters of the source rock, and hydrocarbon generation modeling experiment, hydrocarbon generation amount and oil and gas resource are estimated with genetic mothod. The oil migration and accumulation coefficient refers to the result of new round of resource evaluation in China. The coefficients of oil generated in four periods, which are Ma, Ma, Ma and 30-49Ma, are 13%, 10%, 9% and 7%, respectively. The coefficients of oil generated in four periods are 12, 10, 8 and 6, respectively. Though the values are not completely accord with the actual situation, the oil/gas generated in different periods has different contribution for the oil/gas accumulation. The oil and gas generated earlier undergone several geologic events, diffused more easily, and had less contributions to the reservoir formation. of the Wenchang Formation facies, including drilling and exploration data and highquality seismic data. Sedimentary facies study is analyzed in detail based on the recovery of tectonic inversion, and the range of lake basin and hydrocarbon source rock are further determined. 7. CONCLUSIONS The inversion of rift basins during the faulting period greatly changes the sedimentary pattern, bringing difficulties to the research on sedimentary facies and directly affecting overall evaluation of resource potential. More satisfactory results can be achieved only by organic combination of the inversion structure recovery and sedimentary facies study.inversion structure study is made mainly by trend analysis of seismic reflection, erosion recovery, sedimentary facies study and other methods. The following understanding is acquired by research on the second scene inversion

21 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number structure of Zhuqiong Movement, Wenchang Formation sedimentary facies and hydrocarbon generation in the Huizhou sag. 1. The Huizhou sag experienced strong inversion after sedimentation of the Wenchang Formation. Tectonic inversion of Huidong low uplift changed the sedimentary pattern of the Wenchang Formation. The lake basin was big, and LF13 low-lying and HZ24 low-lying were connected during the sedimentation of the Wenchang Formation. 2. During the sedimentation of the Wenchang Formation and during the strong faulting period of Huizhou sag, it was dominated by a semi-deep lake and shore-shallow lake sedimentary environment, with LF13 low-lying to HZ14 low-lying forming the center of the lake basin. 3. Based on the research on sedimentary facies, thickness and organic matter abundance of main hydrocarbon source rock of Wenchang Formation, the hydrocarbon generation amount was estimated and the resource potential was revaluated. Compared with previous estimations, the hydrocarbon generation amount was greater. It was estimated that the resource amount of the Wenchang Formation is t. It is considered that the deep part of Huizhou sag still has great exploration potential with a large amount of oil and gas resources remaining in the Zhu I depression. ACKNOWLEDGEMENTS This research is supported by China National Offshore Oil Corporation (CNOOC), China national oil and gas resource investigation & assessment and strategic selection (2009GYXQ02-06) and The Fundamental Research Funds for the Central Universities and National Natural Science Foundation of China (Grant No ). REFERENCES Athy L.F., Density, porosity, and compaction of sedimentary rocks. Am Assoc Pet Geol Bull 14(1), Cai Y.W., The hydrocarbon generation in Zhu I depression, the Pearl River Mouth Basin (unpublished data). Cham berlain R.T., The Appalachian folds of central Pennsylvania. Journal of Geology 18(3), Cham berlain R.T., The building of the Colorado rocks. Journal of Geology 27(4), Chen C.M., Shi H.S. and Xu S.C., Oil and gas accumulation conditions for Tertiary reservoirs in Pearl River Mouth basin (east). Scientific press, Beijing. pp Dahlstrom C.D.A., Balanced cross sections. Canadian Journal of Earth Sciences 6(4), Deng H.W., A new school of thought in sequence stratigraphic studies in U.S.: high-resolution sequence stratigraphy. Oil & Gas Geology 16(2), Deng H.W., Guo J.Y., Wang R. and Xie X.J., Tectono-sequence stratigraphic analysis in continental faulted basins. Earth Science Frontiers (China University of Geosciences, Beijing; Peking University) 15(2), 1-7.

22 410 Resource potential assessment of series of strata with few drilled wells in the Huizhou Sag, the Pearl River Mouth Basin, South China Sea Deng H.W., Wang H.L. and Ning N., Sediment volume partition principle: theory basis for high-resolution sequence stratigraphy. Earth Science Frontiers (China University of Geosciences, Beijing) 7(4), Deng H.W.and Cross T.A., High resolution sequence stratigraphy principle and application. Geology Press, Beijing. pp Fu N., Li Y.C. and Wang J. R., Oil-source correlation in the Western Huizhou sag. China Offshore Oil and Gas 15(5), Gidds A.D., Balanced cross-section construction from seismic sections in area of extensional. Journal of Structural Geology 5(2), Gong Z.S., Neotectonic movement and hydrocarbon accumulation in petroliferous basins, offshore China. Oil & Gas Geology 25(2), Jiang H., Wang H., Xiao J., Lin Z.L., Lü X.J. and Cai J., Tectonic inversion and its relationship with hydrocarbon accumulation in Zhu-3 depression of Pearl River Mouth basin. Acta Petrolei Sinic 29(3), Li J.S. and Lin C.M., Forming pattern of self-generating and self-preserving reservoirs in reversal anticline structure. Acta Petrolei Sinic 27(2), Li P.L., Cenozoic structural movement of Pearl River Mouth basin. China Offshore Oil and Gas 7(6), Li W., Summary of reconstruction of strata denudation thickness. China offshore Oil and Gas 10(3), Li Z.S. and Guo B.J., Hydrocarbon potential in the eastern part of Pearl River Mouth basin. China oil and gas (Geology) 11(6), Lin H.Z., Tang Z.Y. and Chen D.X., The new progresses of comprehensive evaluation of the source rocks and oil & source rock correlation in the east part of the Pearl River Mouth Basin (unpublished data). Long G.S., Shi H.S. and Du J.Y., An analysis of creation conditions for Miocene stratigraphic and lithologic traps in Huizhou area, Pearl River Mouth basin. China Offshore Oil and Gas 18(4), Luo Q.S., A discussion on the inverted structure and hydrocarbon accumulation in Taibei depression. Petroleum Exploration and Development 24(5), Nie X.Q., Zhang S.F., Shi H.S., Du J.Y., Zhu R., Luo M. and An J.S., Analysis of lithologic hydrocarbon reservoir accumulation of sequence 2~4 in Huizhou depression of Pearl-river Mouth basin. Journal of Oil and Gas Technology (J JPI) 33(4), OGRSRC (Oil and Gas Resources Strategic Research Center, Ministry of Land and Resources) Oil and gas resource evaluation in China. China Land press, Beijing. pp Shang K.Y., Jia, J.Y., Zhou, Z.Y., Liang, L.F., Quantitative estimation of erosion for Xihu depression: sonic velocity analysis. Offshore Oil 3, Shi H.S., Zhu J.Z., Jiang Z.L., Shu Y., Xie T.J., Wu J.Y., Hydrocarbon resources reassessment in Zhu I depression, Pearl River Mouth basin. China Offshore Oil and Gas 12(1), 9-14.

23 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Tang Z.Y., Ma Z.C. and Dai J.X., The characteristics and evaluation of the source rocks in the WenChang and EnPing formations in the east part of the Pearl River Mouth Basin (unpublished data). Wang F.G., Mei L.F., Shi H.S., Yu S.M. and Wu M., Structural styles of Paleogene in Zhu1 depression of Pearl River Mouth basin. Geotectonica et Metallogenia 32(4), Wang G.C., Hydrocarbon prospect of positive inversion structures in China Offshore basins. China Offshore Oil and Gas (Geology) 12(3), Woodward N. B., Boyer S.E. and Suppe J., An outline of balanced crosssections, 2nd edition. Department of Gelogical Sciences, University of Tennessee, Knoxville Yang J.F., Hong T.Y. and Xu J.Q., Application of trend analysis method in denudation recovery in the inner Junggar basin. West China Petroleum Geosciences 2(1), Yang J.M., The second resources evaluation of Zhu I depression, the Pearl River Mouth Basin (unpublished data). Zhang G.C. and Xu H., Inversion structures in relation to oil and gas field distribution in Songliao basin. Acta Petrolei Sinic 17(2), Zhang Q.Y., Deng H.N., Oil-gas systems in the east part of Pearl River Mouth Basin. Oil & gas geology 17(4), Zhao H.G., Liu C.Y., Weng W.F., Gui X.J., Yue L.P., Wang J.Q. and Liang M.Y., Structural reverse and its significance to oil and gas in the east and west parts of Ordos basin in the Neogene. Acta Petrolei Sinic 28(6), Zheng T.F. and Fu Y.X., Type and distribution characteristics of inverted structures in Jianghan basin. Petroleum Exploration and Development 24(6), Zhou Y.C., Chen X.K. and Cai D.Q., The discussion about the targets in the half-grabens of the Pearl River Mouth Basin (unpublished data). Zhu H.Q., Pang X.Q., Jiang Z.X. and Dong C.H., Studying methods and application of the hydrocarbon accumulation coefficient. Earth Science (Journal of China University of Geosciences) 32(2), Zhu J.Z., Shi H.S., Deng H.W., Wu J.H., Shu Y. and Ding L., Geochemistry of source rocks in Paleogene sequence of Huizhou depression, Pearl River Mouth Basin. Natural Gas Geoscience 18(5),