Structural Delineation Followed by Hydrocarbon Imprints of Dhodak Gas Field, Central Indus basin, Pakistan

Transcription

1 Structural Delineation Followed by Hydrocarbon Imprints of Dhodak Gas Field, Central Indus basin, Pakistan Urooj Shakir 1, Muyassar Hussain 2, M. Asif 1, Mumtaz Ali Khan 1, M. Fahad Mehmood 1, M. Khubaib A. 1, Masood Anwer 1 1 Department of Earth and Environmental Sciences, Bahria University Islamabad, Pakistan. 2 LMK Resources, Islamabad, Pakistan. Corresponding Author mhuroojshakir@gmail.com Abstract Dhodak field is located in eastern Sulaiman Range, Central Indus Basin, characterized by North most culmination of the Safed Koh anticlinorium. The objective of study is to portray the petroleum geology by studying subsurface structures with help of reflection data and to delineate the characteristics of reservoir through Petrophysical analysis. This article highlights the hydrocarbon presence and potential of Dhodak-05 which is found in the structural trap provided by the faults. The research is carried out using seismic lines and the well log data of Dhodak-05 and other available literature. Two-way time and depth contour maps obtained as a result of interpretation have confirmed the presence of Anticlinal pop-up structures. Studies have revealed Pab Sandstone of Cretaceous and lower Ranikot of Paleocene age acting as a reservoir rock. Ghazij shale of Eocene age is acting as a regional seal. The lower Ranikot and Pab Sandstone were evaluated in order to obtain results for hydrocarbon potential. There were two Zones identified in lower Ranikot and Pab Sandstone. Zones in Lower Ranikot have depths ranges from m and m respectively. Zone 1 in Pab Sandstone ranges from m having thickness of 9 m whereas Zone 2 has thickness of 5 m. On the basis of high effective porosity and low Volume of shale, Pab Sandstone is considered potentially valuable. In the context of well interpretation it should be noted that there are small Zones available for Hydrocarbon perspective in both lower Ranikot and Pab Sandstone. Keywords Dhodak field, Tight reservoir, Seismic, Zindapir, Anticlinorium. INTRODUCTION Dhodak Field is situated on the eastern margin of Sulaiman Range and it is the North most culmination of the Sufaid koh anticlinorium, (Humayon et al., 2012), about 80 km north of Dera Ghazi khan city. Field is positioned between / 3.26 // N,70 24'26.25"E to / // N, 70 20'9.23" E as shown in Figures 1& 2. The first part of the paper describes exploration history, the structural features by means of seismic interpretation and literature review. Second part summarizes the hydrocarbon potential of Ranikot (Paleocene) Formation and Pab (Cretaceous) Sandstone by petrophysical analysis of selected Zones. Exploration History The area has been under active exploration since early seventies. This field was discovered in May 1976 by Oil and Gas Development Company Limited (OGDCL) and was brought on regular production in December 1994 (With this discovery, Pab sandstone of cretaceous age was established as gas/condensate reservoir in the Sulamain Range (Anwar et al., 2012). To date, 08 wells have been drilled and 07 wells are gas/condensate producers. Exploration activities in the eastern Sulaiman range date back to 1925 when Burmah Oil Company started some geological studies. Then in the late fifties Pakistan Shell oil company and joint venture of PPL and POL conducted geological surveys. OGDC started its exploration activities in Dhodak and surrounding areas in A further detailed geological mapping was again conducted over Dhodak structure in1973 and Dhodak well-1 was proposed on the basis of this surface geological information (Humayon et al., 2012). METHODOLOGY Seismic data is a substantial source for the understanding of subsurface structural trends. For this purpose 2D Seismic lines have been interpreted to get stratigraphic and structural insight of subsurface. Nature of the seismic lines along with the well point is shown in the base map (Figure 5).In order to interpret the structure; first step is to tie well with seismic section which acts as a bridge between seismic and geological markers. This task is completed with the help of existing well data of Dhodak-05, a time-depth(t-d) chart was generated using time velocity information of well from which depth of each reflector is taken. Then using the formula S = VT/2, depth of the reflectors are calculated and correlated with the well depth. Three horizons are marked on the given seismic section on the base of continuity and strong character of the reflectors. After correlation of the data i.e. from seismic to well tie, the three reflectors are named as: 1. Upper Ranikot 2. Lower Ranikot 3. Pab sandstone Fig. 1 Location on Pakistan provincial map (GSP, 2003). Dhodak area highlighted on google earth. Page 23

2 Database The research is carried out using 2D seismic lines 805- SK-20, 805-SK-18, 795-SK-05R, 805-SK-19, 805-SK-5R, 795-SK-07 and the well log data of Dhodak-05 as shown in Figure 5. GEOLOGY AND TECTONICS The varied geodynamic settings in Indus Basin through geological time resulted in widespread deposition of Infra Cambrian-Eocene source and reservoir rocks of both carbonate and clastic origin along with different stratigraphic and structural traps as a result of different episodes of transtensional-transpressional regimes and salt tectonics (Khan et al., 1986, Malik et al., 1988, Raza et al., 1989, Raza et al., 1990, Soulsby & Raza et al., 1989, Ahmed and Ali,1991, Bannert et al.,1992, Ali et al., 1995, Iqbal et al., 2008, Afzal et al., 2009). The left and right lateral transpressional regime related to wrench tectonics in the east and west gave rise to the development of Sulaiman Fold Belt in Late Tertiary (Ali et al., 1995). In the east, the left lateral en echelon folds and associated thrust faults manifests the wrench related thick-skinned tectonic features on surface, while the west is dominated by the right lateral en echelon folds and positive flower structures in the sub-surface. Wrench related positive flower structures and en echelon folds having hydrocarbon pools have been reported in the eastern part of Sulaiman Fold Belt by Ali et al., 1995, Bannert et al., 1995, Iqbal et al., 2008, Peresson and Daud, According to Iqbal et.al., 2008 the Sulaiman Foredeep acts as a kitchen for the anticlines in the area and the Fold Belt has fast uplift rate in the Late Tertiary giving rise to the barren and under filled structures. The rocks exposed in the Sulaiman Fold Belt are ranging in age from Triassic to Tertiary (Raza et al., 1989; Hunting Survey Corporation, 1960, Baker and Jackson, 1964, Kazmi and Jan, 1997, Shah, 2009, Bannert et.al., 1989). In Zindapir anticlinorium Triassic Alozai Formation has also been drilled (OGDCL, 1989). In Hinterland older rocks are exposed while the Foreland has progressively younger. The total thickness of sediments ranging from meters (Kamel et al., 1982) with almost 7000 meters thickness of Mesozoic and early Tertiary rocks (Raza et al., 1989). According to (Raza et al., 1989) the Paleozoic rocks covering the basement are not exposed and have been drilled in the adjoining region of Punjab Platform. The Precambrian-Permian stratas are assumed to be present in the subsurface (Humayon et al., 1991, Jadoon, 1992, Jadoon et al., 1994). The boundaries of Zindapir Anticlinorium are marked by Sulaiman depression and Barthi Syncline in the east and west respectively. Approximate area of 6000 sq. km is covered by ZindaPir anticlinorium exposing Eocene age rocks in the core of Afiband, Dhodak and Rodho anticlines whereas Paleocene rocks in the Zindapir Anticline. The proven reservoir rocks of Zindapir anticlinorium comprises of i.e., Chiltan Formation (Jurassic), Lower Goru (Early Cretaceous) and Pab Formations of Late Cretaceous, Ranikot Formation and Dunghun Limestone of Paleocene age. (Nazeer et al., 2013) Fig. 2 Tectonic map showing tectonic features, location of study area and divisions of Indus Basin (Modified after Kadri, 1995 Dhodak field is located on the eastern margin of Suleiman Fold and thrust belt where rocks of Cambrian to Oligocene are present. The basement gets deeper towards the Fold Belt, wells drilled on the eastern portion (Figure 2b) of the fold belt have not penetrated below the Triassic sequence. In Dhodak, Cretaceous is represented by Parh, Mughalkot, and Pab formations (Humayon et al., 2012). Cretaceous shales of Sembar, Goru and Mughal Kot Formations are widespread, thick and contain abundant organic matter and are acting as a source rock in the area of Dhodak owing their deposition to the slope environment providing anoxic conditions for the preservation of organic material. Sembar has been identified as the primary source rock for much of the Indus Basin and mainly contains type-iii kerogen, capable of generating gas, although the presence of type-ii kerogen has also been noted (Wandrey et al., 2004). In Central Indus Basin (Sulaiman sub Basin) Pab Sadstone of Upper Cretaceous age is acting as a reservoir in Dhodak gas/condensate field. Seal rock intervals are available for all reservoir horizons of the Sulaiman Fold belt. Ghazij Shale is acting as a seal rock in the study area. The general stratigraphy has been shown in Figure 4. SEISMIC INTERPRETATION Data quality of the seismic sections was very poor and Fault correlation and horizon identification was difficult on some sections. Constant misties have been observed in data and were removed prior to interpretation using interactive way because bulk misties analysis is unable to remove it due to large misties in different seismic sections. Three horizons were marked named as top Upper Ranikot, top Lower Ranikot and top Pab Sandstone. Top Lower Goru and Top Chiltan were not delineated because Dhodak-05 well was not penetrated deep up to that level. However Dhodak-Deep-01 well which is drilled relatively deeper was not provided for this research. Moreover, the low resolution of seismic data at deeper level is another obstacle for the demarcation of Lower Goru and Chiltan Limestone. Upper Ranikot Formation is of Paleocene age and it is Page 24

3 represented by green color on seismic section. Lower Ranikot is marked as second horizon on seismic section and it is colored blue. Pab Sandstone of Cretaceous age is marked and represented in Golden colored reflector. The discontinuity in the reflector represents the faults. The slight movement in the strata is clearly observed with very less throw in the faults. Two faults, F1 (frontal thrust) and F2 (back thrust) dipping in the NE and SW directions develop the popup structure (Figures 6a & 6b). Time and Depth Structural Mapping For making the contour maps, time and depth values are plotted against the latitude, longitude in the software kingdom. Average velocities taken from the velocity functions are used for depth conversion. Time and depth contour maps of lower Ranikot and Pab Sandstone are shown in Figures 7a, 7b, 7c and 7d respectively. The shallow portion bounded within the thrust faults marks the popup structure delineated through subsurface seismic. The upthrown blocks of the both formations in the depth contours have the value in the range of the depths encountered in the borehole. The structure encountered in Lower Ranikot Formation is approximately at 1470 m while it is approximately at 1615 m in Pab Sandstone. Petrophysical interpretation of Dhodak-05 The objective of Well interpretation is to get the petrophysical attribute of well Dhodak-05 at reservoir level. Following methodology is adopted to get the required results. Raw Logs Data The raw log data was acquired from Directorate General of Petroleum Companies (DGPC). The data comprised the raw log curves. The wire line log of Dhodak-05 well has been evaluated. Suite of logs includes gamma-ray log (GR), Neutron log, Sonic and resistivity log. Zone of Interest First of all the clean Zones were marked using the Gamma Ray Log. Then the log trends of Neutron and Density logs have been recognized at clean Zones. Crossovers are observed between Neutron and Density log curves. These crossovers are sign of hydrocarbons in the particular Zone. Resistivity curves also have shown the presence of hydrocarbons. The depth of investigations for the Zones of interest has been shown in Table -01. Table I. Zone of Interest Formations Depth(m) Thickness(m) Drazinda Pirkoh Limestone Sirki-Domanda Habib Rahi Limestone Rubbly Limestone Ghazij Shale Dunghan Upper Ranikot Lower Ranikot Pab Sandstone Fig. 4 Generalized stratigraphic column of central Indus basin (Kadri, 1995) Fig. 3 Raw Logs Data Average parameters for the well have been computed and based on these parameters Zones were ranked according to their Hydrocarbon Potential. The general stratigraphy encountered in well Dhodak -05 has been shown in Figure 3. Lithology Confirmation Gamma ray log was used to differentiate between clean and dirty Zones. High Gamma ray Values depicts dirty Zone whereas low Gamma ray values gives the indication of clean lithology. Page 25

4 Volume of shale is calculated by using the Gamma Ray Log. As in the quantitative assessment of the shale content, it is assumed that the radioactive minerals are absent in clean rocks and are compared to the shaly rocks. To calculate the volume of shale we have used the following formula: Volume of shale (VSH) = GR log - GR max/ GR max - GR min Where, GR log = Gamma ray log reading. GR max =Maximum Gamma ray deflection. GR min = Minimum Gamma ray deflection. Using the above-mentioned formula volume of shale of four marked Zones is calculated. Gamma ray lies in correlation track as shown in figures 8a and 8b. Fig. 5 Base map showing seismic lines and well point. Fig. 7(a) Time contour map of Lower Ranikot Formation Fig. 6(a) Interpreted section of line GO-795-SK-06. Fig. 7(b) Depth contour map of Lower Ranikot Formation Fig. 6 (b) Interpreted section of line GO-795-SK-07 Calculation of Shale Volume (Vsh) Porosity Calculation Porosity in Pab Sandstone is mostly secondary and is preserved in selective stratigraphic horizons. Porosity appears to have been influenced mainly by cementation and clay/shale content of the sands (Moghal et al., 2012). Mechanical compaction, authigenic cements like calcite and quartz reduced the primary porosity of the sandstones, whereas, dissolution of feldspar and volcanic grains have Page 26

5 enhanced and produced secondary porosity up to 15.53% (average 2.77 to 10.61%) (Memon, 2011). Ranikot Formation also exhibits good porosities. In this Research, porosity values at different depths were computed by using Neutron and Density Logs. Then average porosities were calculated by combining Neutron and Density values. Porosity was calculated by using following formula. Effective Porosity= Vsand * Porosity avg Avg. Porosity = (Density porosity + Neutron porosity) /2 Saturation of Hydrocarbon is shown in last track along with saturation of water as shown in figures 8a and 8b. Fig. 8(a) Well interpretation of Dhodak-05. Zone 1 and Zone 2 of Lower are shown highlighting the Hydrocarbon Potential of Lower Ranikot Fig. 7(c) Time contour map of Pab Sandstone Fig. 7(d) Depth contour map of Pab Sandstone Saturation of Water (S W) The fraction of pore space containing water is and it is denoted by Sw. Archie Water Saturation Saturation of water calculated is shown in last track in Figures 8(a) and 8(b). Saturation of Hydrocarbons (S H) Calculation of the saturation of hydrocarbon is a very significant step, because the reservoir potential to yield hydrocarbons is checked. The Formula for the calculation of hydrocarbon saturation is, S h = 1-S w Fig. 8(b) Well interpretation of Dhodak-05. Zone 1 and Zone 2 are shown highlighting the Hydrocarbon Potential of Pab Sandston RESULTS Two ways time and depth contour maps obtained as a result of interpretation have confirmed the presence of Anticlinal pop-up structures. In Lower Ranikot Formation Zone 1 range from 2019m to 2029m in depth, having total thickness of 10 meters whereas in Zone 2 Lower Ranikot thickness is 25 meters. Zone 1 in Pab Sandstone ranges from Page 27

6 m having thickness of 9 meters whereas Zone 2 in Pab Sandstone thickness is 5 meters. Average Volume of shale in Zone 1 and Zone 2 of Lower Ranikot is 24% and 22% respectively whereas in Pab Sandstone Zones Volume of Shale is 9% and 4% which decrease in Volume of shale as we move from lower Ranikot to Pab Sandstone. Average Effective Porosity of Zone 1 and Zone 2 in Lower Ranikot is 6% and 10% while in Pab Sandstone, effective Porosity is 9% and 7% in Zone 1 and Zone 2 respectively. Average Saturation of water and saturation of gas in Zone 1 of Lower Ranikot is 35% and 65% and in Zone 2 it is 26% and 74% respectively. In case of Pab sandstone, average water saturation and gas saturation is 27% and 73 % for Zone 1 and 22% and 78% for Zone 2 respectively. DISCUSSION The present research work in Dhodak area exhibits the hydrocarbon presence and potential of Dhodak-05 which is found in the structural trap provided by the faults. Two-way time and depth contour maps obtained as a result of interpretation have confirmed the presence of Anticlinal popup structures. Studies have revealed that the main source rock in the study area is Sembar shale of Cretaceous age whereas Pab Sandstone of Cretaceous and lower Ranikot of Paleocene age act as reservoir rocks. The study is conducted with the aid of seismic and wireline log data. The structure studied through geology and literature review, clearly delineated on the seismic data with two Faults, at the level of Paleocene and Late Cretaceous age. CONCLUSIONS The study reveals that the project area being so old, still shows positive signs of presence of Gas reserves. The Dhodak structure is an asymmetrical anticline with a steeper eastern limb which is disturbed by a west verging back thrust. Well interpretation carried out at lower Ranikot level and at Pab level is showing the fair potential but only few Zones have shown good porosities and hydrocarbon results. Time and Depth structure maps confirm the presence of valid structure at Paleocene and late Cretaceous level. Well interpretation of Zones reveals that reservoirs of Paleocene and Late Cretaceous are tight reservoirs. RECOMMENDATIONS Full suite of Well logs data and high resolution seismic data is recommended in order to reveal more about the structure. 3D seismic data is highly desirable in order to delineate structures correctly. ACKNOWLEDGMENT The authors are quite obliged to Department of Earth and Environmental Sciences, Bahria University Islamabad, for the merciful conduct, capable direction and scholastics feedback throughout the whole study. We are also thankful to DGPC for providing data for this research. REFERENCES [1] Afzal.J., T.Kuffner., A.Rahman., and M.Ibrahim, (2009). Seismic and Well-log Based Sequence Stratigraphy of The Early Cretaceous, Lower Goru C Sand of The Sawan Gas Field, Middle Indus Platform, Pakistan. 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