Geochemical Characterization of Jurassic Coals in Minghe Basin, Shaanxi-Gansu Province (P.R. China)

LI, XINYU, University of Oklahoma, Norman, OK

Crude oils have been found in Minghe basin in drill wells and petroleum could be observed in the Jurassic coal seams. The potential source rocks have not been determined yet, but the coal might be a possible candidate. Four coal samples from different locations and two oil samples have been studied by means of vitrinite reflectance [R(0)] measurement, maceral composition, Rock Eval, stable carbon isotopic analysis, GC, and GC/MS analysis.

The vitrinite value R(0) of two coals is around 0.54 which defined them as marginal-low mature. The maceral composition of the coals is homogeneous, dominantly composed of exinite and vitrinite. Rock Eval hydrolysis revealed the low portion of S(1) [S(1)/[S(1)+S(2)] < 10]. The EOM (extractable organic matters) ranges from 25,003 ppm to 58,795 ppm. The low ratio of bitumen/asphaltene and high percentage of NSO fraction suggest the low maturity of all coal samples. The C(13) value vs. Pr/Ph relation reflect efficiently the paleoenvironment when the source rock and oils were formed and it suggested that the coal was formed in swamp facies.n-Alkane distribution of the coals was characterized by odd-even predominance with C(25) main peak while the oils show no carbon number predominance and the main peak is at C(17) or C(19). Biomarker characteristic of high plant input like diterpanes, De-A-lupane, Oleanane were found in coals. Abundant triterpanes and bb-steric hopanes were identified in one coal sample, while in the oils triterpanes of hopane type are dominant. Steroids exist in very low concentration in all coal samples.

It was concluded that the Jurassic coals in Minghe basin were not mature enough to generate the oil. Attention should be paid to the Paleozoic rocks that could be correlated with the source rocks in the neighboring basin--Shanganning basin.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)