The Lower Jurassic Datta Formation, western Salt Range, Pakistan, comprises three facies associations: (1) channel belt facies association (CBFA), (2) channel margin, and overbank facies association (CMOFA), and (3) lagoonal facies association (LFA). A cyclic fining-upward trend in the succession is represented by basal quartzose conglomerate/pebbly sandstone, through coarse to fine quartzose sandstone to siltstone and shales/claystone, which contains some carbonate accumulation. Two prominent depositional sequences are recognized in the Datta Formation with the lower high and upper low magnitude cycles. The Datta Formation thus represents a thick sedimentary succession and in the study area, i.e., western Salt Range, mainly channel belt, flood plain and/or delta top facies are exposed. The palaeocurrent analysis shows that the source area with acidic plutonic rocks laid to S-SE in the Indian shield, aravalies or older sedimentary rocks of the Indus Basin (i.e., Khewra, Tobra and Warchha formations). A tentative stratigraphic correlation of the Datta Formation with the lower Jurassic Lathi Formation, India invites further work in parts of India, which will elaborate the extent of the Datta Formation in the Greater Indian peninsula and develop palaeogeographic setting for this Lower Jurassic deltaic rock unit.
The assessment of the gas generation
potential of highly to overmature
shale is key for shale gas evaluation, but it is always fraught with
challenges. In this study, immature shale samples with type-I kerogen
were investigated for their oil expulsion efficiencies (EEs) by means
of pyrolysis in sealed gold tubes to investigate the influence of
EEs on the gas generation process. Experimental results were then
applied as an analogue to evaluate the gas generation potential of
the Lower Cambrian Niutitang shale reservoir in South China. The results
demonstrate that the gas yields of C1–C5 alkanes obviously decrease with increasing EEs and are a result
of thermal cracking gas caused by a decrease of the oil retention.
The change of the gas drying index (i.e., C1/C1–5) indicates that EasyRo of 1.81% calculated from pyrolysis temperature
indicates the start of a significant cracking of heavy hydrocarbon
gases (C2–5). Before, the generated gas is mainly
wet gas, whereas at higher maturities, C2+ alkanes are
substantially cracked into methane. The iC4/nC4 and iC5/nC5 ratios exhibit a gently increasing
trend, which may result from the catalytic role of clay minerals.
Considering the semiquantitative statistical analysis of solid bitumen,
the EEs of the Lower Cambrian Niutitang shale in South China were
calculated. The gas generation potential of the Niutitang shale of
two selected wells is in the range of 4.21–37.62 m3/t and 2.53–11.15 m3/t, respectively, whereas the
gas contents of these two wells are low. This mismatch indicates that
the gas accumulation in the Niutitang shale reservoir is related to
preservation conditions rather than based exclusively on the generation
potential of the shale. This study offers a conceptual approach to
evaluate the gas generation potential of organic-rich shale after
hydrocarbon expulsion and enhances the accuracy of the prediction
of highly mature and overmature shale gas resources in South China.
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