Study of Structural Styles and Hydrocarbon Potential of Rajan Pur Area, Middle Indus Basin, Pakistan

Transcription

1 Study of Structural Styles and Hydrocarbon Potential of Rajan Pur Area, Middle Indus Basin, Pakistan Shabeer Ahmed Abbasi 1, Zakiullah Kalwar 2, Sarfraz Hussain Solangi 3 1 Oil and Gas Development Company Limited, Islamabad 2 Pakistan Oil Limited, Islamabad 3 Center for Pure and Applied Geology, University of Sindh, Jamshoro Abstract - This research encompass the study of structural styles and evaluation of the hydrocarbon potential of Kotrum and Drigri anticlines located in Rajanpur Area, Midddle Indus Basin of Pakistan with the approach of geophysical data integration. The study area is situated between the Sulaiman Foldbelt on the west and Indus River in the east. It is an anticlinal fold, located to the south-east of Sakhi Sarwar anticline and separated from a prominent syncline. The structure has narrow elongated crest, with axis running in SSW-NNE direction. In the east structure is bounded by a gentle syncline. Structural Styles are trending East- West and perpendicular to tectonic transport and stress direction and the base of the structures gradually dipping Eastward beneath the deformation frontal part in Eastern Sulaiman Fold Belt. Middle Indus Basin can be divided into Foreland, Sulaiman fold belt and a broad foredeep. Sulaiman represent a blind thrust front which suggests that all frontal folds of the fold belt are cored by blind thrust. The deformation of frontal part of Sulaiman Lobe represents the passive roof duplex stacked beneath the frontal passive roof thrust. The passive roof thrust which has a back thrust sense of motion and extends into interior of Fold belt. Left lateral Kingri Fault separates Eastern and Central Sulaiman fold belt. In Central Sulaiman fold belt the deformation front moved further towards fore deep as compared to Eastern Sulaiman. Two wells (Kotrum-01, Drigri-01) have been drilled in the study area with the objective to determine the potential of oil and gas in Habib Rahi Limestone of Eocene age, Dunghan Limestone of Paleocene age and Pab Sandstone of cretaceous age and role of structural styles in hydrocarbon potential of study area. Kotrum-01 well was drilled to its T.D of 4798m. Besides fishing and side tracking, tight hole conditions, high pressure and losses of circulation were also encountered. During production testing Pab sandstone were tested but abandoned found. Drigri-01 well was drilled to its T.D 3250 m. RFT was carried out at different points but all points showed no pressure / seal failure and the well was plugged and declared abandoned. Kms new 2D seismic for the investigation of hydrocarbon potential and to identify new prospect. Two exploratory wells have been drilled in Rajanpur E.L. area by OGDCL. The first exploratory well (Kotrum 1) was drilled in 1977, down to 15,742 feet in Pab sandstone (Cretaceous) in the fourth sidetrack. OGDCL tested Eocene Habib Rahi Limestone, Paleocene Dunghan carbonates, Ranikot and Khadro sands and Cretaceous Pab sands. All the tested formations produced formation water with no indication of hydrocarbons. OGDCL again acquired the area and drilled Drigri 1 well, on northeren part of the Kotrum structure, into Gazij Formation (Eocene) with well TD at 10,663ft (KB), no indications of hydrocarbons were recorded and the well was declared abandoned. Location of Rajnpur block is shown in Fig.1 and Fig. 2. Fig. 1 Location of study area Key words: Pab, Sulaiman fold belt, seismic interpretation, Hydrocarbon Trap. I. INTRODUCTION In order to define the scope of Rajanpur structure and find out the potentials 2D seismic data interpretation has been done on previous available data. Processing of 914-RPR, 924-RPR and 954-FZP vintages were done by OGDC and C vintage was by GSI. Acquisition of 914-RPR, 924-RPR and 954-FZP were done in 1994, 1992 and1995 respectively and acquired by OGDCL party number four. POL acquired 212 Fig. 2 Study Area Seismic Block Page 36

2 II. TECTONIC SETTING / GEOLOGY Middle Indus Basin can be divided into Foreland and thrust belt of Sulaiman and a broad fore deep. Sulaiman represent a blind thrust front which suggests that all frontal folds of the fold belt are cored by blind thrust. The deformation front of Sulaiman Lobe represents the passive roof duplex stacked beneath the frontal passive roof thrust. The passive roof thrust which has a back thrust sense of motion extend into interior of Fold belt. Left lateral Kingri Fault separates Eastern and Central Sulaiman fold belt. In Central Sulaiman fold belt the deformation front moved further towards fore deep as compared to Eastern Sulaiman. Structures are trending east-west roughly perpendicular to tectonic transport direction. Base is gradually dipping eastward beneath the deformation front in Eastern Sulaiman Range. A) BASIN DEVELOPMENT The breakup of the Gondwana land occurred in the Jurassic and involved NE-SW extension, with further NE- SW extension involved in rifting between the Indian and Madagascar Plates in the Cretaceous. Passive margin condition existed on the edges of the Indian and Eurasian Plates during the Late Cretaceous, although subduction is likely to have occurred and preceded ophiolite emplacement at the end of the Cretaceous. B) BASIN INVERSION It is likely that the start of the collision between the Indian and Eurasian Plates was marked by basin inversion, with reverse reactivation of basement and basin- controlling faults. However, this is not supported by seismic data available in the region due to imaging issues prevailing in the data. C) STRUCTURATION Collision between Indian and Eurasian Plates during Tertiary initiated folding and thrusting. The first phase of thrusting involved ophiolite emplacement along the western margins of the Indian Plate (west of the area under review). The main phase of thrusting in the Sulaiman region was from Late Miocene to the present Fig. 3. D) STRATIGRAPHY OF THE AREA The Rajanpur area contains Permian to Recent clastics and carbonates, and possibly the same Cambrian succession as occurs in the Kohat-Potwar Fold Belt. The succession generally is younger in the southeast than in northwest. Intraformational sedimentation breaks are pronounced in Permian and Jurassic, while Cretaceous Tertiary unconformity is regional. Erosion in some parts of the basin is so deep that it has exposed the Jurassic rocks Fig. 4 Fig. 3 Fig. 4 Regional Tectonic Framework of Study area Generalized Stratigraphic Column of Study area Page 37

3 E) STRUCTURAL STYLES The Rajanpur area is situated between the sulaiman range on the west and Indus River in the east. It is an anticlinal fold, located to the south-east of Sakhi Sarwar anticline and separated from it by a prominent syncline. The structure has narrow elongated crest, with axis running in SSW-NNE direction. In the east structure is bounded by a gentle syncline. F) PETROLEUM GEOLOGY Sulaiman Foldbelt and Fore deep region has a proven petroleum system. Sembar and Lower Goru shales of Early Cretaceous age are proven source rocks. Additionally, shale beds in Eocene Habib Rahi and Pirkoh Formations are reported to be organic rich (upto 8% in Habib Rahi and 4% in Pirkoh) and in early phase of oil-window in the region of interest. Paleocene Ranikot and Dunghan Formations are also candidate source (TOC upto 1.5wt %) although not well documented. In Sulaiman Foldbelt carbonate and sandstone reservoirs of Jurassic to Eocene age are mainly gas producer, and in some places Cretaceous and Eocene reservoirs are gas/condensate producers. Thick sealing shales are present in Upper Goru (Cretaceous), upper Ranikot claystone (Paleocene) and Ghazij shales (Eocene) in addition to intraformational shale/claystone seals in entire stratigraphic column. Thrust anticlines of Oligocene and younger are proven hydrocarbon traps in foldbelt region however in the foredeep region none of the identified structures is commercial producer. G) SOURCE ROCK Data for Jurassic and older rocks is rare in the region under discussion. Results of outcrpos and well cuttings geochemistry reveal Sembar and Lower Goru shales as the main source intervals. Sembar Formation is found throughout most of the Sulaiman region and its source quality varies widely. Fair good organic carbon contents are found in outcrops in some areas such as Mughal Kot ( %), Loralai (upto 2.14%). High TOC is reported in Giandari 1 well (upto 4.33%) and in Shahdani 1 well (upto 2.91% in an undifferentiated Goru and Sembar Formaton). In these sections, however, present generartive potential is low. Hydrogen indices (HI) range between 1 and 51 in well Shahdani 1. The organic matter within the Sembar Formation is mainly of Type II/III of marine origin but can be strongly influenced on a local scale, particularly in the eastern part of the basin, by terriginous input such as in the well Zindapir -1 where organic matter is Type III. H) SEAL Significant shale/claystone intervals exist in the Rajanpur E.L. region which can act as effective top seal in their respective plays. Sembar shale(~600ft) at top of Chiltan reservoir is proven top seal in the entire region for the Chiltan Play. I) TRAPS All the hydrocarbon bearing strctures are thrust anticlines with 4- way closure formed by compressional as well as transpressional forces resulting from Indian Plate collision with Eurasian Plate. These structures are Late Oligocene in age and younger eastwards. According to the depositional model of Ranikot and Sember/Goru, There is also a possiblity of stratigraphic traps within Ranikot, Sembar and Goru formations. J) RESERVOIRS Pab and Ranikot sands are considered as primary reservoirs as these formations are producing gas/condensate in the nearby fields, e.g. Rodho Field is some 92km north of Kotrum 1 well. Secondary targets may include, from older to younger, Chiltan, Goru, Sembar, Dunghan, Habib Rahi and Pirkoh; all are proven reservoirs in the different parts of the Sulaiman Region. III. SEISMIC INTERPERTATION By using old and newly acquired seismic data, a detailed interpretation was carried out. Five horizons Habib Rahi, Ranikot, Pab, Mughalkot and Lower Goru were mapped. It was found that there exists only one anticline in the Area where two wells have already been drilled and the wells were found to be at optimum locations. Fig. 5 Interpretation of Strike Line 914-RPR-07 Kotrum-Drigri anticline is an elongated doubly plunging anticline trending NNE-SSW and is bounded on the east and west by high angle gentle throw faults. The eastern limb of the anticline is gently dipping while western limb of the anticline is comparatively steeper. The infill new seismic data Page 38

4 has confirmed northern limit of the anticline. To the north of Drigri- 1 well the eastern limb of the anticline diminish at Ranikot and deeper levels therefore, Kotrum-Drigri anticline form a monocline to the north. From the seismic sections it is obvious that youngest rocks are involved in folding and the structure appear to be very young Fig. 5 Fig. 8 Geoseismic Crossection of Main Strike Line Fig. 6 Interpretation of Strike Line 914-RPR-05 Fig. 7 Interpretation of Line 914-RPR-03 Fig. 9 D View of Seismic attribute on Pab reservoir Page 39

5 reported and the well was abandoned without testing. POL acquired 212 L.Km new 2D seismic to identify any new prospect but old and new seismic does not reveal any new structure. The effectiveness and presence of source and reservoir rocks are favorable in the area of interest. Cretaceous Sembar and Goru Shale beds with good organic content (TOC upto 4%, Type II/III) are currently in gas generation window in the area. Source rock intervals are also reported in Eocene Kirthar Group (TOC upto 8%, Type II). Moderate reservoir quality Paleocene Lower Ranikot and Cretaceous Pab sandstone beds (porosity upto 8%) exist in the area. Fig. 10 Two way time map on top Pab V. CONCLUSIONS Two wells (Kotrum-01, Drigri-01) have been drilled in the study area with the objective to determine the potential of oil and gas in Habib Rahi Limestone of Eocene age, Dunghan Limestone of Paleocene age and Pab Sandstone of cretaceous age and role of structural styles in hydrocarbon potential of study area. Kotrum-01 well was drilled to its T.D of 4798m. Besides fishing and side tracking, tight hole conditions, high pressure and losses of circulation were also encountered. During production testing Pab sandstone were tested but abandoned found. Drigri-01 well was drilled to its T.D 3250 m. RFT was carried out at different points but all points showed no pressure / seal failure and the well was plugged and declared abandoned. Fig. 11 Depth map on top Pab Fm IV. DISCUSSION Two exploratory wells have been drilled in Rajanpur E.L. area. OGDCL drilled first exploratory well (Kotrum 1) in 1977, down to Pab sandstone (Cretaceous) with total depth (TD) of 15,742ft (KB) in the fourth sidetrack. OGDCL tested Eocene Habib Rahi limestone, Paleocene Dunghan carbonates, Ranikot and Khadro sands and Cretaceous Pab sands, all the tested intervals produced formation water and no indication of hydrocarbons was recorded. OGDCL again acquired the area and drilled Drigri 1 well, on northeren part of the Kotrum structure, into Gazij Formation (Eocene) with well TD at 10,663ft (KB), no hydrocarbon indications were REFERENCES [1] Ahmad, A., Ahmad, N., Wali, N., Manshoor, S.A., Source potential and maturity trends of Lower Permian sediments in the Punjab platform and adjoining areas, Pakistan.Annual Technical Conference, Pakistan,Proceedings, [2] Nazeer, A., Solangi., Sarfraz, H., Brohi, I.A.,Usmani, P., Napar, L.D., Jahangir, M.,Hameed, S., Ali, S.M., Hydrocarbon Potential of Zinda Pir Anticline, Eastern Sulaiman Fold Belt, Middle Indus Basin, Pakistan, Pakistan Journal of Hydrocarbon Research. 22, 23, [3] Bannert, D., Cheema, A., Ahmed, A., 1989, Interpretation of LANDSAT-MSS imagery of the Sulaiman and Kirthar ranges in western Pakistan. Hydrocarbon Development Institute Pakistan-BGR, Internal Report, 83, [4] Bannert, D., Raza H.A., The segmentation of the Indo- Pakistan Plate. Pakistan Journal of Hydrocarbon Research, 4(2), [5] Bannert, D., Iqbal, Helmcke, D., Surface and subsurface evidence for the Existence of the Sulaiman Basement Fault of the North Western Indian Plate in Pakistan. Second south Asia Geological congress- Colombo, Srilanka, 21. [6] Bender, F.K., Raza, H.A., Geology of Pakistan. Borntraeger, Berlin, Stuttgart, 414. [7] Hasany, S.T., Aftab, M., Siddiqui, R.A., Refound Exploration opportunities in Infracambrian and Cambrian sediments of Punjab Platform, Pakistan. Annual Technical Conference, Islamabad, Pakistan. Proceedings, Page 40

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