The Research Of Lithofacies Palaeogeographic Characteristics And Shale Gas Resource Potential Of The Lower Cambrian Series In The Upper Yangtze Region

With the growing domestic demand of petroleum resources, looking for new energy resource is imperative for sustainable development of domestic oil&gas. According to the exploration&development situation of non-renewable resources in the world, shale gas is identified as an important alternative energy to conventional hydrocarbon resources.In upper Yangtze Region, the widely distributed Lower Cambrian marine shale with large thickness,high organic material content and maturity is very favourable for accumulation of shale gas. Therefore, proceeding exploration of shale gas of Lower Cambrian in upper Yangtze region has a big significance for finding alternative energy.This paper takes some theories such as modern sedimentary theory and shale lithofacies paleogeographic analysis technology as guidance, considering the lower Cambrian Shuijingtuo Formation shale stratum in Middle Yangtze region as the research target. Based on field outcrop profiles, in combination with drilling and logging data interpretation, observation of rock slices and masses of analysis-testing data research, the typical profiles and single well sedimentary facies comprehensive bar chart and well tie sedimentary facies stratigraphic correlation chart of the research area were drawed up. On the basis of the above, combining regional tectonic sedimentary background and evolution characteristics, the Early Cambrian lithofacies paleogeographic diagram was established and deeply discussed. Then by using organic geochemistry data, organic carbon content and organic matter maturity equivalence value maps of researched area were compiled. Integrating with some research factors such as the reservoir conditions and storage condition, the shale gas predicted favored region diagrams were compiled.Based on the observation of field outcrops and cores, the figures and types of sedimentary facies of Lower Cambrian Niutitang Formation in upper Yangtze region are studied in detail, and the lithofacies palaeogeographic characteristic is also discussed.The Niutitang Formation in study area mainly develops a set of clastic rock deep water shelf sedimentary system. Four subfacies such as clastic rock deep water continental shelf, clastic rock shallow water continental shelf, carbonate ramp and deep water basin are identified, in which clastic rock deep water continental shelf is mainly argillaceous continental shelf microfacies. In the plane, from northwest to southeast, clastic rock deep water continental shelf, clastic rock shallow water continental shelf and deep water basin were developed sequentially, of which chromocratic shale is mainly developed in clastic rock deep water continental shelf.Through laboratory analysis methods, according to geochemistry data, organic-riched shale in research region is analysised, which suggests that distribution range of TOC of the Lower Cambrian Niutitang Formation mud shale is0.21%～15.39%in upper Yangtze region, average is3.14%. The main organic type of primary hydrocarbon generating matrix is Ⅰ, some is Ⅱ1kerogens.Organic matter generally is in high maturity, in general, in high mature-over mature stage.By analysis about microscopic characteristics by rock slice, scanning electron micro scope, rock X-ray diffraction, clay X-ray diffraction of mud shale in the study area, the changes of horizontal and vertical differences of the mud of the mineral and shale brittle clay minerals are learned. The result shows that brittle mineral is mainly the quartz mineral, followed by clay mineral, carbonate at least. Clay mineral content is average distribution is10.37%～47.37%, and average content is36.6%. The main clay minerals are I/S, illite (I), chlorite(C), and the illite content is the highest. From planar distribution characteristics of mineral components, brittle mineral content ranges in46.89%-88.54%, average of56.82%.On the basis of the above, the reservoir characteristics of the lower Cambrian mud shale in research are researched. The porosity of shale ranges in0.71%～3.7%, and average value is2.06%, with permeability is0.0052×10-3μm2～0.244×10-3μm2, average of0.12×10-3μm2. Reservoir space mainly includes micropores and microcracks, the pore micropores are mainly composed of orgaic pore, kerogen network, mineral pores(mineral surface, intergranular space, dissolved pore and matrix pore) and other spacesIn the upper Yangtze region, shale gas accumulation are mainly controlled by shale thickness distribution, the buried depth, rock and mineral composition, organic the characteristics, the tectonic activities and so on. With the researched of mud shale gas generating ability, reservoir characteristics and the preservation condition, according to the division standard by the zone of enrichment shale gas, shale gas favorable rich zone is divided into vision zone and advantageous zone. Integrating with the mud shale thickness, organic carbon content, organic matter maturity shale buried depth, earth’s surface, fracturing, preservation conditions and so on, drawing up the shale gas distribution forecasting figure of the Lower Cambrian Niutitang Formation in upper Yangtze region. The result shows that the favorable region of shale gas of Niutitang Formation is mainly distributed in weiyuan、northern Guizhou and eastern Chongqing-western Hubei.