Characterization of selected petroleum source rocks and reservoir rocks of Bengal Basin (Bangladesh) based on geochemical, petrographical and petrophysical methods / Md. Farhaduzzaman

Md. Farhaduzzaman, Md. Farhaduzzaman (2013) Characterization of selected petroleum source rocks and reservoir rocks of Bengal Basin (Bangladesh) based on geochemical, petrographical and petrophysical methods / Md. Farhaduzzaman. PhD thesis, University of Malaya.

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Abstract

Nine gas fields and two coal fields of Bengal Basin, Bangladesh are studied in the current research based on geochemical, petrographical and petrophysical methods. The Surma Group of Bengal Basin is composed of the Bhuban and Boka Bil Formations. Both formations are composed of sandstones and shales interpreted to have been deposited in a deltaic to shallow-marine environment. This is the most important geological unit of the basin since the entire hydrocarbon accumulations so far discovered in Bangladesh is found within these sandstones. These two formations also contain shale intervals with important source rock potential. Together with coal and coaly samples, shales and sandstones of the Surma Group are the key subjects of the present study. Around 151 core samples of shale, sandstone and coal were investigated for this project. Shale and sandstone cores were chosen from ten wells of respective nine gas fields. Five bore holes of the Barapukuria and Dighipara Basins were selected for coal, carbargillite and mudstone sampling. All shale, coal and coaly samples are subjected to a Source Rock Analyzer (SRA). After SRA screening, different standard geochemical methods (e.g., PyGC, GC, GCMS, AAS, TGDTA, EA and proximate analysis) were employed for the current study. Many shale and coal and coaly samples are examined with UV-facilitated microscope for maceral and vitrinite reflectance analyses. Poor to fair quality source rock potential is estimated for the studied Bhuban and Boka Bil shales. The organic matter found in both formations is thermally immature to early mature for hydrocarbon generation. The organic matters of the analyzed shales are derived from land plants of terrestrial environmental settings with minor contribution from marine sources. The siliciclastic sandstone samples are studied using both petrographic microscope, SEM and XRD for reservoir quality and diagenetic control analysis. Very good to excellent reservoir quality is evaluated by the present study. Together with petrography, petrophysical analysis is also carried out for one well (Rashidpur 4) and four potential hydrocarbon bearing zones are identified.The depositional environment and hydrocarbon generation potential is another focus of the present study, emphasizing the biomarker characteristics of Permian coals of the Barapukuria and Dighipara half-graben basins. In this project, organic facies distributions within the Permian succession are investigated focusing on coals, carbargillites and mudstones in the half-graben basins. The studied bituminous B rank coal consists dominantly of inertinite macerals followed by vitrinite and liptinite. The various facies models used commonly indicate forest swamps with mixed oxic-anoxic conditions under terrestrial settings with periodic flooding. Organic facies characteristics suggest that the coals are dominantly terrestrial with minor contributions from marine sources. The carbargillites correspond to a mixture of terrestrial and marine sources, whereby the mudstones being terrestrial with no marine influences. Very good hydrocarbon generation potential is estimated for the analyzed coal and coaly samples, ranging from fair to excellent. The carbargillites possess good potential for both oil and gas and followed by coals (mainly gas with minor oil) and mudstones (predominantly gas). Along with %Ro and Tmax, the presence of exsudatinite, fluorinite and solid bitumen suggests the analyzed coal and coaly samples have already expelled hydrocarbons.

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