In The Yangtze Region Cambrian Sequence Stratigraphy

On the basis of description of the Cambrian peried stratigraphic sections and sequence stratigraphic characteristics in the Mid Yangtze area, combined with sequential boundary characteristics of other series of strata, 8 markers for distinguishing sequence boundaries are summarized , which include palaeoweathering crust, solution cavity-typed colluvial rocks, anhydrite-dissolved breccias, reactivated fluvial, slope-faciesed breccias, bird-eye structure, scum-shaped layers, scour water channel deposits above platform. All these markers are typical and direct, which can be confirmed with sections.Regional data are used in sequence correlations of subsequences (or super-sequences)and of subsets of the subsequences (sedimentary sequence) in the Mid Yangtze area, construction of 4 maps of lithofacies paleogeography for the Cambrian sequence in a subsequence system tract considered as a single unit and illustration of the horizontal expansion and the vertical evolution of lithofacies paleogeography of various units.By analyzing the sedimentary thickness of every subset of subsequence tract, a concept is proposed, consisting of three types of sequence textures and three sequence factors, which are composed of transgressive (1ST), level-high (HST) and transitional (HT) textures, and causes that form them are roughly analyzed. The Cambrian sequence in the Mid Yangtze area is composed of the level-high, transgressive and transitional sequences, reflecting sedimentary characteristics of a stable cratonic area. A curve with the S0 value is unusually similar to that of sedimentary facies, so one can suppose that the S0 value can reflect changes of the sea level. If the suppose is correct, then a qualitative analysis of the sea level will be transformed into a quantitative analysis.The Analysis of the hydrocabon-generated horizon, rock types andorganic geochemical indicators of the source rocks suggests that the transgressive system tract in the subsequence is the main hydrocarbon-generated source rocks horizon. In the transgressive system tract, the level-high sequence texture is favorable to the development of the source rocks.Depending on various evaluation indicators, an integral approval and projection of the Cambrian source rocks are carried out both vertically and horizontally. In vertical, the Shuijingtuo Formation source rocks are best developed, corresponding to the main body of the 1ST of the SS3. In horizontal, the Chongqing-Hubei-Hunan border area is considered as an area with best-developed source rocks where the hydrocarbon-generated center is located in areas of Enshi-Xianfeng-Sangzhi- Yangjiaping of Shimen. Furthermore, the source rocks have a highest hydrocarbon contribution fraction up to 9. 45% and a highest hydrocarbon-generating intensity of 47.54 X lOVkra2. As a result, the source rocks provide beneficial hydrocarbon-generating conditions for formation of a great oil and natural gas reservoir.