Biomarker signatures of Sinian bitumens in the Moxi-Gaoshiti Bulge of Sichuan Basin, China: Geological significance for paleo-oil reservoirs

- Pyrobitumens with various shapes were widely found in the reservoir rock.
- Pyrobitumens were originated from a highly reduced marine shale environment.
- Sinian Dengying and low Cambrian Qiongzhusi formations are major source rocks.
- Paleo-oil filling pathways were revealed through the study of pyrobitumen.
- Analysis of bitumen was a potential means to study paleo-oil reservoirs.

The unique Moxi-Gaoshiti Bulge of Sichuan Basin in southwestern China provides insights into the biomarker signatures of bitumens and their geological significance for paleo-oil reservoirs in Precambrian strata. Solid bitumens with various shapes occur widely in the voids, solution pores, cracks, stylolites, and veins of reservoir rocks. Biomarker analysis was conducted on bitumens and the associated source rocks by quantitative detection with gas chromatography-mass spectrometry (GC-MS) to constrain thermal maturity, sedimentary environments, bitumen/paleo-oil sources, and the possible paleo-oil filling directions, which are dominant geological factors that control paleo-oil accumulations. Quantitative analysis of biomarkers showed that the bitumens in the Sinian dolomite of the Moxi-Gaoshiti Bulge belong to the same group. The bitumens are highly matured, and their reflectance values (%Rb) are greater than 1.5 with an average of 2.96. These bitumens originated from thermal cracking of oil (pyrobitmen) via a high-temperature process. The biomarker characteristics revealed that the bitumens and related paleo-oil reservoirs originated from a highly reduced but moderately saline marine shale environment. The marine shales of the Sinian Dengying and lower Cambrian Qiongzhusi formations are their major source rocks. The directions of the primary oil-filling pathways in the Gaoshiti and Moxi sub-bulges are from west to east and from northwest to southeast, respectively. These directions indicate that the Deyang-Anyue sag west of the Moxi-Gaoshiti Bulge is the major hydrocarbon source kitchen.