The Bitumen-Source Correlation Study in Majiang Paleo-Reservoir and Kaili Remnant

Yuhong Liao, Ansong Geng,Yunxin Fang, Liangliang Wu, Fang Yuan, Yinhua Pan, and Yijun Zheng
The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China

High-over mature Paleozoic source rocks and paleo-reservoir solid bitumens occur extensively in marine sequences of Southern Guizhou Depression, China. Most of them are at high-over maturity stage and low in extractable organic matter (EOM). It is difficult to get enough biomarkers by Soxhlet extraction, simple pyrolysis or chemical degradation from high-overmature kerogens and solid bitumen. Furthermore, those routine biomarkers in EOM have been severely altered by thermal alteration and lost most original geochemical infromation. Majiang paleo-reservoir is one of the biggest paleo-reservoirs formed in Caledonian period in southern China. The potential source rocks of Sourthern Guizhou Depression and solid bitumens of the Majiang paleo-reservoir are commonly at high maturity with Ro%> 2.0%. Majiang paleo-reservoir is believed to have experienced both severe biodegradation and subsequent severe thermal alteration. There also are some severely biodegraded oil seepages in the neighbouring Kaili area, namely, Kaili remnant reservoir. Kaili remnant reservoir has not experienced deep burial but severe biodegradation due to continuous uplift.

Compared to Soxhlet extraction, yields of EOM are significantly increased by hydropyrolysis (HyPy), and the covalently bound biomarkers released by HyPy can satisfy the requirement of instrumental analysis. Previous studies also showed that the geochemical parameters for source and maturity based on biomarkers released from the thermally altered bitumen residues by HyPy are insensitive to the degree of thermal alteration. Our recent work (Liao et al., 2012) also suggests that source and maturity biomarker parameters, based on the products of HyPy, remain valid for bitumens which have suffered both biodegradation and severe thermal maturation.

Thus covalently bound biomarkers released by HyPy from high-over mature potential source rock kerogens of the Majiang paleo-reservoir and solid bitumens in Ordovician-Silurian strata are used in bitumen-source correlation. Bulk isotope of kerogens and bitumens, together with the distribution of triaromatic steroids in EOM and oil seepages are also used. The results suggest that solid bitumens in Ordovician-Silurian strata are from the lower Cambrian source rocks. The severely biodegraded oil seepage in Ordovician-Silurian strata of Kaili remnant reservoir are also generated from the lower Cambrian source rocks. But Ordovician-Silurian strata of the neighbouring Kaili remnant reservoir may be charged twice by the same lower Cambrian source rocks because these oil seepages show obvious mixing characteristics of both biodegraded oil and non-biodegraded oil. Moreover, the isotopic values of bulk bitumens and n-alkanes isolated from the bitumens are very light and similar to the values of the lower Cambrian kerogen. All the results above indicat that lower Cambrian source rocks are the main source of Ordovician-Silurian strata in both Majiang paleo-reservior and Kaili remnant reservoir.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013