Abstract
An organic petrology evaluation and a determination of solid bitumen reflectance [Formula: see text] were completed for organic-rich Triassic Yanchang Formation mudrocks ([Formula: see text]) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boquillas Formation), we suggest that the original organic matter in the organic-rich samples examined for our study may have been type I/II kerogen with hydrogen index values of [Formula: see text] TOC.
Highlights
Unconventional gas resources in lacustrine shale and mudrock in the continental basins of northern China have received significant attention in the past few years following successful development of unconventional marine “shales” in North America (Lin et al, 2013)
As part of a larger study of the petroleum geology of the Yanchang Formation, we present in this contribution the organic petrology and solid bitumen reflectance values (BRo) of peak oil mature Chang 7 and other Yanchang mudrocks and integrate these data with the results from Rock-Eval programmed pyrolysis
The total organic carbon (TOC) concentrations are in the range of 3.7–14.7 wt% for unextracted Yanchang mudrocks with the highest TOC content occurring in Chang 7 (Table 1)
Summary
Unconventional gas resources in lacustrine shale and mudrock in the continental basins of northern China have received significant attention in the past few years following successful development of unconventional marine “shales” in North America (Lin et al, 2013). Lacustrine mudrocks in China are characterized by higher clay concentrations and lower thermal maturity than most North American shale gas systems (Tang et al, 2013a, 2013b). The Upper Triassic Yanchang Formation is a lacustrine unit consisting of repetitive transgressive-regressive sequences of fluvial, shoreface, and deepwater sediments deposited in fluviolacustrine settings (Zhao et al, 2015). It has been divided into 10 members, called Chang 1–10, from oldest to youngest, respectively.
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