Tracking multiple paleo oil charges in the Precambrian dolomite reservoirs of the central Sichuan Basin, China

Abstract

In the Precambrian dolomites in the central Sichuan Basin, a substantial paleo-oil system previously existed, which has since evolved into a dry gas system that is currently in place. The genesis of this paleo-oil system remained poorly understood, leaving uncertainties regarding the timing of oil charging and associated source rocks. Integration of Re–Os geochronometry with basin modeling allows temporal-spatial constraints on the ancient petroleum system to be established.Two texturally distinct generations of reservoir bitumen were identified. Late-generation bitumen yielded a geologically meaningful Re–Os age of 189 ± 20 Ma (MSWD = 10.4, 2σ), distinct from the previously documented Re–Os ages such as the 414 ± 44 Ma reported by Shi et al. (2020). This new age, along with the ∼414 Ma, aligns within uncertainties with the two relative timings derived from our independent basin model (ca. 450–390 Ma and 260–180 Ma), revealing two temporally distinct oil generation-migration events. The date of ∼189 Ma also lends support to the previous oil charge timing inferred from fluid inclusions. We evaluated the imprecisions of these two Re–Os ages and considered them to result from a narrow spread in 187Re/188Os, and variations in initial Os isotopic compositions (Osi) due to the giant size of the petroleum system and a long duration of oil generation from source kitchen over a large geographical area in this play. Effects of asphaltene precipitation, due to uplift-induced decompression of the migrating oils, may also have contributed to the scatter. Separating the samples based on Osi produced a higher precision age (184.1 ± 5.7 Ma, Model 1) with an MSWD of 0.97.The first oil charge is relatively weak as shown by the basin model and observed bitumen volumes, and it had not been well preserved due to significant reservoir exhumation by Caledonian tectonism. Consequently, the younger oil charge is considered to be the principal precursor to the current dry gas accumulations. Os isotope histories for four potential source units were constructed using publicly available Re–Os data. Comparisons between bitumen and rock Os compositions tend to support the lower Cambrian Qiongzhusi Formation as the predominant source unit for the two major paleo-oil charges. However, contributions from the Duoshantuo, Dengying, and Maidiping source rocks can not be precluded. Overall, these new observations allow temporal-spatial constraints to be placed on the complex evolution, from paleo-oil generation to destruction and dry gas generation.