Recognition of in situ oil cracking in the Precambrian-Lower cambrian petroleum systems of sichuan basin, southwestern China

Determining the place of paleo-oil cracking is crucial to understanding the formation mechanism of the gas reservoirs in the Precambrian-Cambrian strata. For the old strata the attainable information is limited because most paleo-oil have been cracked to gas at high thermal maturity, such as the Anyue giant gas field in the Moxi-Gaoshiti Bulge of Sichuan Basin, Southwestern China. In this study, the place of paleo-oil cracking was discussed based on solid bitumen found in the reservoirs. Numerous solid bitumen from the deep boreholes in the Precambrian Dengying (Z(2)dn(2) and Z(2)dn(4)) and Lower Cambrian Longwangmiao (E(1)l) formations of the Moxi-Gaoshiti Bulge were analyzed. Elemental and carbon isotope analyses and optical microscopic observations, such as Raman spectroscopy, scanning electron microscopy, and electronic probe, were performed on these solid bitumen samples. The obtained parameters, including the reflectance of the solid bitumen, parameters I(D)/I(G) and S(D)/S(G) from the Raman spectra, the compositions and carbon isotope of gas, burial history, and the predicted "equivalent vitrinite reflectance Ro" (EqVRo) for reservoirs, jointly support that solid bitumen are epigenetic bitumen in reservoirs other than primary bitumen in source rocks. This study provides a relatively solid evidence of in situ cracking of paleo-oil and the thermochemical sulfate reduction reaction between paleo-oil and dolomite reservoirs. Pyrite were found both in carbonate rocks and argillaceous rock, and the sulfur in pyrite is perceived as a pathway of H2S formation. The results provide a basis for understanding the formation mechanism of the deep huge gas field that was derived from paleo-oil cracking, and may contribute to future petroleum exploration in deep Precambrian-Lower Cambrian systems.