Genesis and evolution of reservoir bitumen in Xiaocaoba paleo-oil reservoir, Qianbei Depression

The solid bitumen in the Xiaocaoba paleo-oil reservoir of Devonian in the Qianbei Depression fills solution caves, intracrystalline pores and fractures of dolomites containing biodetritus. The solid bitumen, with an equivalent R_o of 4.08%-4.14%, hardly dissolves in organic solvent. The n-alkanes have a high n-C_21^-/n-C_22⁺ value at the range of 1.5-9.0. Most bitumen samples have a higher abundance of cheilanthanes relative to hopanes, and also a relatively higher abundance of C₂₁-pregnane and C₂₂-pregnane relative to C_27-29-sterane. The bitumen and bitumen inclusions do not fluoresce, and the bitumen has no absorption peaks of aliphatic series-containing groups or oxygen-containing groups in the infrared spectrum. The physical, optical and geochemical characteristics of bitumen indicate that it formed in the process of thermal cracking under high temperature. The sterane and terpane parameters and the δ¹³C of the bitumen are obviously different from those of sources rocks in the Cambrian Qiongzhusi Formation and Silurian Longmaxi Formation. But the δ¹³C of the bitumen are very close to that of the Upper Paleozoic sources rocks, indicating that the bitumen derived from these source rocks. Ancient oil reservoirs are interpreted to have been trapped in the Middle-Late Triassic period. The bitumen formed in the Early Cretaceous period while oil reservoirs were deeply buried and then uplifted to ground level in the Yan-shanian and Himalayan periods.