Discovery of oil bitumen co-existing with solid bitumen in the Lower Cambrian Longwangmiao giant gas reservoir, Sichuan Basin, southwestern China: Implications for hydrocarbon accumulation process

Abstract

Natural gas in the recently discovered Lower Cambrian Longwangmiao giant gas reservoir of the Sichuan Basin in southwestern China has been considered to be from oil-cracking and sourced from the underlying Lower Cambrian Qiongzhusi shales, similar to the underlying Neoproterozoic Dengying giant gas reservoir. However, geological considerations and differences in geochemistry between the Longwangmiao and Dengying gases/bitumens imply different scenarios. Organic petrology, geochemistry and trace element analyses of widely occurring bitumen were conducted to evaluate this hypothesis. Results show the coexistence of solid bitumen and oil bitumen in this reservoir for the first time, which implies a new hydrocarbon accumulation process and associated exploration strategies. The two types of bitumen were termed Type A solid bitumen and Type B oil bitumen, which were sourced from the Lower Cambrian Qiongzhusi Formation shale and the Lower Silurian Longmaxi shale, respectively. The Type A solid bitumen shows little fluorescence and its color under plane-polarized light is almost black. This indicates that the Type A solid bitumen is likely of high maturity. It typically occurs in dissolution pores, and accounts for approximately 90% of all bitumen in the reservoir. In contrast, the Type B oil bitumen is brown–black in color under plane-polarized light, and yellow–green under fluorescence light, indicating that it is mature. It occurs in the residual spaces of the solid bitumen, and represents approximately 10% of the total bitumen content. This suggests that Type A bitumen charged the reservoir earlier than did the Type B bitumen. During the Indosinian Epoch of the Triassic, oil generated from the mature Qiongzhusi Formation migrated into the Longwangmiao and Dengying reservoirs. Following this, the Dengying reservoir reached the temperature threshold for oil cracking since the Late Cretaceous, and it is suggested that oil could have started to crack to gas, which is the critical moment for the gas formation. However, the Longwangmiao oils had not been cracked at this stage because the reservoir temperature had not reached the threshold. Later, during the Himalayan Epoch, the gas in the Dengying reservoir, as well as some kerogen-cracking gas from the Qiongzhusi shales, migrated into the Longwangmiao reservoir along faults. Gas gravity drainage and phaseseparation took place in the Longwangmiao reservoir, forming the Type A solid bitumen and giant gas accumulation. Later, oil generated from the Longmaxi shales migrated into the Longwangmiao reservoir, forming the Type B oil bitumen. The case here is an example of a multi-stage oil and gas accumulation. Future exploration targets should include Middle–Upper Cambrian reservoir rocks.