Abstract:

A set of important source rock is developed in the Lower Cambrian Maidiping Formation (∈₁m)-Qiongzhusi Formation (∈₁q) in the Yangtze region, which is of great significance for the petroleum exploration of the Sinian-Paleozoic. In order to clarify source rock conditions in the exploration field of the Sinian-Cambrian with sparse wells in Sichuan Basin, the systematic analysis of wells and outcrop samples, logging evaluation and seismic inversion are conducted to characterize the distribution of ∈₁m- ∈₁q source rocks by sequences and sections, define the boundary of petroleum system with the source kitchen of this set of source rocks, and propose the favorable exploration zones. The study results show that: (1) The total thickness of ∈₁m source rocks mainly ranges in 5-200 m, and that of the high-quality source rocks is 0-100 m in the Middle-Upper Yangtze region, with the development area limited to the intracratonic and marginal rifts. (2) The total thickness of ∈₁q source rocks is between 50-450 m, and that of the high-quality source rocks is in the range of 5-250 m. The ∈₁q source rocks have the characteristics of “three sections”, that is, the high-quality source rocks are generally developed in the first member and lower part of the second member, while the poor-quality source rocks with low organic carbon content are developed in the third member. Among them, source rock in the first member of ∈₁q has a thickness of 10-300 m, with the similar distribution characteristics to that of ∈₁m, which is the product of the early transgression in the Early Cambrian. The source rock in the second member of ∈₁q has a thickness of 30-200 m and a wide distribution area in the Middle-Upper Yangtze region, which is the product in the maximum transgression period. The thickness of the poorquality source rock in the third member of ∈₁q ranges from several meters to tens of meters, and the distribution area is similar to that of the second member. (3) The thickness of source rocks of ∈₁m- ∈₁q is controlled by multiple factors, such as the distribution of intracratonic and marginal rifts, development of synsedimentary faults, paleogeomorphology at the end of the Sinian, as well as the later corrosion. (4) The Lower Cambrian source rocks show great hydrocarbon generation potential with hydrocarbon generation intensity of (20-140)×10⁸m³/km².There are two large-scale hydrocarbon generation centers, leading to two petroleum systems with hydrocarbon kitchen of this set of source rocks, i.e., the Sinian-Permian composite petroleum system in Deyang-Anyue intracratonic rift and the Sinian-Ordovician composite petroleum system in Chengkou-Badong-Wufeng intracratonic rift. Finally, seven favorable exploration zones are proposed in the two composite petroleum systems.