Abstract
Upper Triassic coaly and lacustrine source rocks complicate efforts to determine the source of hydrocarbons in Sichuan Basin. Total organic carbon analyses, pyrolysis experiments, petrological examinations, and gas chromatography and gas chromatography–mass spectrometry determinations were conducted on coals, carbonaceous mudstones and mudstones collected from two outcrop sections and cores of nine wells. Results revealed that the abundant organic carbon content will prolong the hydrocarbon generation cycle for coals and then the hydrocarbon generating capacity of coals will be enhanced by salinization, thereby contributing bacteria and algae microorganisms into humic coal of the Xujiahe Formation. Compared with mudstone, coal with the same maturity has a stronger adsorption effect on free hydrocarbons. When Ro is greater than 1.35, coal still has a strong hydrocarbon generation ability. The stable water column stratification and euxinic bottom water conditions are evidenced from the lower Pr/Ph and the higher gammacerane indices. The mixing of sea water has been proven by the existence of 4α,23,24,-trimethylcholestanes.
Highlights
Previous studies have demonstrated that coal measures have significant liquid hydrocarbon potential (Chen et al, 2001; Van Koeverden et al, 2010)
In the opinion of Horsfield (Horsfield et al, 1988), rocks with a total organic carbon (TOC) ranging between 25% and 75% are coals, while the TOC value of mudstones is less than 5%
The S2 values of coals are always greater than mudstones in the range of the temperature of maximum generation (Tmax) from 430C to 570C, and the situation is similar between carbonaceous mudstones and mudstones(Figure 2(b)); these indicated that the abundant organic carbon content prolongs the hydrocarbon generation cycle for source rocks
Summary
Previous studies have demonstrated that coal measures have significant liquid hydrocarbon potential (Chen et al, 2001; Van Koeverden et al, 2010). Condensates are extensively produced in the terrigenous Upper Triassic Xujiahe Formation in the central and southern parts of Sichuan Basin (Huang et al, 2014). The purpose of this study is to compare the potential source rocks of mudstone and coal samples from the Xujiahe Formation of the Upper Triassic in the Sichuan Basin, including an evaluation of hydrocarbon generation potential, the maceral composition of organic materials and the biomarker distribution. Analytical data was used to interpret the depositional environments of the source rock. These results will assist with future explorations in transitional sea and land strata
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