2D PETROLEUM SYSTEM MODELING TRANQUEBAR SUB-BASIN, CAUVERY BASIN

Sushma Rawat, A. Kaur S. Tiwari, D. Kumar, S. Sunderaman, , S Jaiswalrawat_sushma@ongc.co.in

Keywords

Petroleum Systems Modeling (PSM), Source Rock, Thermal Maturity, Transformation ratio, Critical Moment

Summary

ABSTRACT

Cauvery Basin is a Pericratonic rift basin, on the EastCoast of India. The basic architecture consists of en-echelon asymmetric half-grabens, separated bybasement horsts. It has 7-8 km thick syn-rift and postrift stratigraphic succession deposited as aconsequence of basin evolution since Late Jurassic(145Ma). An understanding of basin evolution and therelated petroleum system is an essential step inhydrocarbon exploration and exploitation. InTranquebar sub-basin, the flanks are HC rich localesbut other prospects drilled within the sub-basin aredry, in spite of having good TOC and porosityretention even on deep burial. 2D- Petroleum SystemModeling study along a geological cross section wasthe basis for paleohistory reconstruction and toassess hydrocarbon prospectivity. It traces the GMEcycle through geologic time and leads to betterunderstanding of hydrocarbon proclivity of the area.

Introduction

The journey of exploration for hydrocarbon in Cauverybasin began in 1958.The first wildcat well was drilled onKaraikal high in 1964. Presence of hydrocarbon (thoughnon-commercial) in this well had given an impetus tointensify exploration. However commercial success wasnot met until 1984. Several oil & gas fields werediscovered on the onland & offshore parts of Cauverybasin. PY-3 structure towards east in the offshore extensionof Tranquebar sub-basin was discovered 1988, whereasKali and Kuthalam oil and gas fields (onshore) withinNannilam and Andimadam Formations in the NW werediscovered in 1998. Mayavaram-1 gave definitive gasshows. All other prospects are barren. Tranquebar issupposed to have one of the best TOC content in CauveryBasin. Reservoir properties are preserved even at 300mdepths (TKD-3 produced water @ 108m3/d) fromNannilam Fm.In view of these set of present geological conditions, it is anenigma why HC exploration results in Tranquebar sub-basin are not fruitful.

Petroleum system modeling was carried out to answerquestions with regard to generation, timing of hydrocarbonexpulsion and accumulation along a 2D seismic transect inTranquebar sub basin. The model also helps to understandwhether the source rock in this area is within the oil or gasgeneration window. Due to the paucity of calibration datain the deepest part of the modeled 2D transect, geochemicalinformation from offset wells has been included in themodel. The calibration wells were selected based on theirproximity to the modeled 2D transect, abundance andreliability of data.

Regional geology and Tectonic setting

Cauvery basin is a pericratonic rift basin, on the East Coastof India. The entire sedimentary column deposited as aconsequence of the basin evolution since Late Jurassic-Early Cretaceous period (145 to 112 Ma), has been termedas “East Coast Super group”. The basin has 7-8 km thicksyn-rift and post rift sediments resting over the basement.The basic architecture of the basin consists of en-echelonasymmetric half-grabens arranged in three trends, separatedby basement horsts. The half grabens/subbasins from southto north are: Gulf of Mannar, Ramnad-Palk-Bay, Tanjore,Nagapattinam, Tranquebar and Ariyalur-Pondicherry. Thebasement horsts/ridges are: Mandapam-Delft, Pattukottai-Mannargudi, Karaikal, Kumbakonam-Madanam-Portonovo(Fig. 1). The dominant basement trend in the basin is NE-SW and is parallel to the regional Eastern Ghat trend.

The Tranquebar sub-basin trending NE-SW, encompassespresent day onland and offshore parts of Cauvery basin. Itis bounded by Karaikal horst to southeast andKumbakonam-Madanam-Portonovo high to the northwest.The sub-basin forms a part of the tri-junction towardssouthwest, where the Tanjore and Nagapattinam sub-basinsmeet Tranquebar sub-basin. The onland part of theTranquebar sub-basin is a type area for the entire litho-stratigraphic succession in the subsurface that portrays thesedimentation history of the entire Cauvery basin.

11th Biennial International Conference & Exposition

2D Petroleum System Modeling across Tranquebar sub-basin, Cauvery Basin

Fig. 1: Tectonic map of Cauvery Basin

Petroleum System Modeling

The petroleum system modeling study gives a betterunderstanding of the potential petroleum systems within thebasin. It illustrates the maturation of a source rock and themigration into potential traps. For the present study, 2 DModule of PetroMod software Version 2013.1 has beenutilized.

Methodology

Standard Workflow for PSM study as provided inPetroMod software is as follows

To carry out PSM study, along 2D seismic transect in theTranquebar sub-basin (Fig. 2), Horizons / Faults weredigitized and gridded to make the basic model geometry(Fig. 3).

Fig. 2: Map showing study area and location of modeled2D transect

COLLECTION / CONDITIONING OFDATA

STRATIGRAPHIC MODEL STRUCTURAL MODEL GEOCHEMICAL MODEL

DATA INPUT

BASIC GEOLOGICAL MODEL

THERMAL MODEL

GENERATION & EXPULSION MODEL (2D/3D)

PETROLEUM SYSTEMS ANALYSIS

CALIBRATION DATA(%VRo, BHT, etc)

11th Biennial International Conference & Exposition

2D Petroleum System Modeling across Tranquebar sub-basin, Cauvery Basin

Fig. 3: Basic Model Geometry showing horizons and faults.

Each formation/layer having lateral variation of faciesacross the modeled 2D transect (Fig.4) is assigned with itsrelevant lithological facies and geochemical propertiespertaining to Petroleum system elements is given in Table-1. The classification of the different source rocks werederived by laboratory data basis and the interpretation ofthe geological history and depositional environment of thearea. The Albian and Cenomanian age source rocks areassumed to be a Type III, generally gas-prone with organicmatter being mainly woody and coaly.

Fig. 4: Lateral facies distributions across modeled 2Dtransect.

Table-1: Facies definition with geochemical parametersacross the modeled 2D transect

Geological age of deposition of different formation/ layeras well as erosion and hiatus events observed in geologicalpast of Tranquebar sub-basin are assigned in the modelled2D transect is given in Table-2.

Table-2: Geological age of deposition and erosion/hiatusassigned in modeled 2D transect

Boundary Conditions

Heat Flow Trends used are different for shelfal area andbasinal part. Paleo Water Depth information was derivedfrom analysis of sediment composition related to theirdepositional environment (e.g. carbonates in water depthless than 50m) and structural evolution (e.g. age of rise ofridges).Sediment Water Interface Temperature for studyarea through geologic time generated inside PetroModsoftware. Boundary conditions (HF, PWD, SWIT) arecalibrated along the modeled section (Fig.4).

11th Biennial International Conference & Exposition

2D Petroleum System Modeling across Tranquebar sub-basin, Cauvery Basin

Fig. 4: Boundary Conditions for Modeling (HF, PWD, SWIT) andTemperature & VRo Calibration

Depending on seal thickness and their absolute depthbreakthroughs could be seen within different migrationscenarios. In ‘closed’ (sealing) fault scenarios, faults mayact as parts of a structural trap. In ‘open’ (non-sealing)faults scenarios hydrocarbon migration could be shownalong faults. The fault definition was assigned based ontectonic history and interpretation of seismic data.

Modeling Results

The modeling results shows that the Upper and LowerAlbian source rocks are in late oil to wet gas generationwindow and have attained Critical Moment approximatelyfrom 114 to 99 Ma and 85 to 76Ma respectively, withpresent day transformation ratio for Upper and LowerAlbian source rock of approximately from 96-85% and 75-69% respectively in basinal part of sub-basin. Cenomaniansource rock falls within main oil generation window andhas attained CM around 16-13Ma with present daytransformation ratio (TR%) in range of 55-38 % in basinalpart of sub-basin (Fig. 5A, 5B, 5C). Most of the generatedhydrocarbon is expelled from the sub-basin via upward andsideways migration and the accumulations observed alongthe modeled section in reserviors of Cretaceous andPaleocene age are mainly structurally entrapped (Fig. 6).

Fig.5A: Maturity of source rocks along modeled 2Dtransect

Fig.5B: Transformation Ratios source rocks along modeled2D transect

Fig.5C: Critical Moments of source rocks along modeled2D transect

11th Biennial International Conference & Exposition

2D Petroleum System Modeling across Tranquebar sub-basin, Cauvery Basin

Fig.6: Vectors showing migration path of expelledhydrocarbon along with accumulations

Conclusions

In the Tranquebar sub-basin , the Lower and Upper Albianand Cenomanian source rocks are mature and have expelledsignificant quantity of Hydrocarbon. Significant part of theexpelled hydrocarban is lost during migration while somehydrocarban accumulation are observed due to structuralentrapment in the modeled 2D transect.

The Albian lower and upper source rocks shows significanttransformation upto 96 % and 85 % respectively while TheCenomanian source rocks shows transformation upto 55 %.The main expulsion peak for Albain lower and uppersource rocks in the Tranquebar sub-basin could beevaluated around 114 Ma to 76 Ma respectively whereasfor Cenomanian source rock around 16-13 Ma.

Miocene tilting might have affected establishedaccumulations and effected some large-scale distribution ofhydrocarbons.

References

Burnham, A. K., 1989, A simple model of petroleumformation and cracking. Lawrence Livermore Lab. Report.UCID 21665, March, 1989

Pande, D.K., 2008, Docket on Petroleum SystemsSequence Stratigraphy, Cauvery basin.

Sinha A.K., Katiyar A.K., Kohli K.B. and Bahuguna S.P.,2006 Vitrinite Reflectance Studies of Exploratory Wells ofCauvery Basin, KDMIPE Unpublished Report.

Wygrala B., 1989, Integrated study of an oil field in thesouthern Po basin, Northern Italy. Berichtekernforschungsanlage julich, 2313: p.217.

Well Completion Reports of Cauvery Basin, ONGCUnpublished Report.

Acknowledgements

Authors wish to thank Dr. D. N. Singh, GGM-HOI-KDMIPE for his encouragement. Authors are thankful toDr. A. K. Parakh, GM (Geology), Head BRG and Shri S.Das, GM (Geology))for their support. Authors are thankfulto Shri A. K. Sinha, DGM (Chemistry) for fruitfuldiscussion and valuable suggestions. We also wish to thankShri G. C. Sati, DGM (Geology) and the geoscientists ofCauvery Group, KDMIPE for valuable discussions.

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