Cambrian lithofacies paleogeographic characteristics of the Upper Yangtze Block: implications for the marine basin evolution and hydrocarbon accumulation of small-scale tectonic blocks in China

The reconstruction of litho-paleogeography plays an important role in understanding the evolution of sedimentary environment, spatial distribution of sedimentary formations and hydrocarbon exploration of basins. In order to reveal the Cambrian paleogeographic setting which is the controlling precondition of the newly discovered Anyue giant gas field in the Upper Yangtze Block and its significances to potential carbonate oil and gas exploration, based on the seismic and borehole-logging data and the geological survey of outcrop section surrounding the Sichuan Basin, the lithofacies and paleogeography of the two Cambrian tectonic sequences were reconstructed, and the characteristics of Cambrian source-reservoir-seal assemblage were analyzed. The results showed that, in the earlier Eearly Cambrian, the tectonic extensional background induced a broken craton basin which had a deep trough cutting the Upper Yangtze Block with north-south direction.The infill of organic-rich sediments in the trough became the kitchen of deep giant hydrocarbonlayers. From the later Early Cambrian to the late Cambrian, the upper Yangtze plate shifted to a relatively stable tectonic setting, and the basins on the Upper Yangtze Block were unified and formed a craton depression basin. The sedimentary formation was characterized by the development of the epeiric tidal wedge and associated salt. The tidal energy in the epeiric sea constrained the macroscopic distribution of the reservoirs. The deposits filling in tectonic active stages were superimposed by infill of the relatively stable intracraton depressions, forming a sequence combination similar to the rift basin which is composed of rift and post-rift sequences. The transection-profile of this type of sequence combination is like a cartoon of the ox-head. The paleogeographic characteristics of the basin, which experienced tectonic active interval and tectonic stability interval alternately, was a major feature of Chinese small-scale blocks linked to their affinity with the big-scale plates and drifting discretely in paleo-oceans in the pre-Late Triassic period. This special was favor for forming potential source-reservoir-seal assemblages in the sedimentary formation which can develop giant hydrocarbon reservoir in intracratonic basins.