Heterogeneity And Controlling Factors Of Organic-rich Shale In Continental Rift Basin

Two cases have been studied, including the fifth shale interval of Hetaoyuan Formation in the Biyang depression and the lower part of Third-Member shale interval of Shahejie Formation in the Zhanhua depression. According to the geological, core and logging data, as well as a large number of analysis and testing data, and application of the theories and methods of sedimentology, petrology and geochemistry, the petrological characteristics of organic-rich shale interval have been studied, the heterogeneity of minerals composition and distribution of organic matter in the mudstone and shale in specific interval have been revealed. On this basis, the controlling factors of heterogeneity of the organic-rich shale in the typical faulted depressions have been investigated and their development models have been summarized. The main results obtained are as follows:1. The formation background and distribution characteristics of the organic-rich shale are revealedThe fifth shale interval of Hetaoyuan Formation in the Biyang depression and the lower part of Third-Member shale interval of Shahejie Formation in the Zhanhua depression as the key analysis object in this paper. The TOC content of the mudstone and shale of the fifth shale interval of Hetaoyuan formation in the Biyang depression is about 2.14%-4.96%, with an average value of 3.27%, the highest value are occur in the deep depression. The main types of organic matter are the kerogen type of ? and ?,which are the partial oil type. The TOC content of the mudstone and shale of lower part of Third-member shale interval of Shahejie Formation in the Zhanhua depression is 1.6%-9.1%, the main types of organic matter are the kerogen type of ? and ?. These high quality hydrocarbon source rocks are characterized by high organic matter abundance, good type and high hydrocarbon-generating ability. The results show that the organic-rich shale intervals in the Biyang depression and Zhanhua depression were formed during the Middle and Late Eocene strong rifting period, although these rifted depressions are widely developed mudstone and shale intervals, but the organic matter enrichment intervals in the shale are formed in the following conditions: ? Developed in the semi-deep and deep lake environments, ? Deposited in brackish and saline water environments, ? Most of organic-rich shale developed in the lake expansion system tract (EST) or early highstand system tract (HST).2. Heterogeneity characteristics of lithofacies of organic-rich mudstone and shale intervals are summarizedIn addition to the development of terrigeneous coarse clastic intervals, the laminae are usually very well developed, such as organic-rich laminae, carbonate-rich laminae and silty laminae. Most of the organic-rich shale lithofacies are located in the calcareous-rich and transition zone. The main lithofacies are calcareous shale with low silicon and low clay and with medium silicon and medium clay. But in different system tracts there are different lithofacies, the lithological variation from calcareous-rich to silicon-rich and clay-rich occur from lake expanding system tract(EST) to highstand system tract (HST), this change indicates that there are strong evaporation at the bottom of EST, and water column deepened, evaporation decreased, and terrigeneous supply enhanced in the period of HST.3. Heterogeneity of mineral compositions of organic-rich mudstone and shale in typical rifted depression are explainedClay minerals in organic-rich mudstone and shale are mainly illite/smectite mixed interval and illite, kaolinite, chlorite and montmorillonite, in which illite/smectite mixed interval and illite content account for the largest proportion of the total, but the chlorite and kaolinite content is very low. Brittle mineral contents are generally higher, which quartz content is the highest, followed by albite, calcite and dolomite. The mineral composition of shale have obvious differences in different system tracts, the content of clay minerals in the lake expanding system tract is slightly lower than that of high system tract, and the content of the brittle mineral in the EST is significantly lower than that of the HST. Compared with marine shales, continental organic-rich mudstone and shale are characterized by high content of brittle minerals with more than 70% in general, and high brittleness index.4. Heterogeneity of organic matter distribution and difference of organic facies are revealedThe distribution of organic matter in mudstone and shale is closely related to lithology, high TOC content is present in calcareous shale, followed by organic carbon content of argillaceous shale, and the lowest organic carbon content of siliceous rocks.In addition, the organic matter content in the EST and early HST are significantly higher than that in the LST and late HST. Three types of organic facies in organic-rich shale interval are identified, namely algae source, shell source and mixed source. Algae source facies can be further subdivided into algal subfacies and algal mud subfacies.Similarly, the distribution of these organic facies are closely related to the system tracts,shell source facies developed in the early stage of the EST, algal source facies developed in the EST and the early HST, and mixed source facies developed in the late HST. In the different parasequences and parasequence sets of the EST interbeds of algal subfacies and algal mud subfacies are usually present.5. The controlling factors of heterogeneity of organic-rich mudstone and shale in the typical rifted depression are explored1) Controlling factors of shale lithofacies heterogeneity. The heterogeneity of shale lithofacies includes both macroscopic and microscopic heterogeneity. Macroscopic heterogeneity includes the variation of sediment grain size, structure and texture,lithology, variation of laminae and its assemble. The microscopic heterogeneity mainly includes TOC content, maceral, mineral content, arrangement of mineral particles and different geochemical indicators. The heterogeneity of shale lithofacies are controlled by the environmental background, including paleostructure, paleoclimate and sedimentary environments, secondly, it is controlled by the source supply and lake level change. Furthermore, late diagenesis also has a certain influence on the heterogeneity of shale lithofacies. In the rifted depressions of Eastern China, organic-rich mudstone and shale and main hydrocarbon source rocks were developed in the Middle and Late Eocene strong rifting stage and arid-humid climatic conditions, this is the primary controlling factors on the formation of organic-rich mudstone and shale.2) Controlling factors of shale organic matter heterogeneity. Shale organic matter enrichment is not only related to the background environment (paleoclimate,paleosalinity), but also to the preservation conditions (especially the redox conditions).In the Fifth shale interval of Hetaoyuan Formation in the Biyang depression, the analysis result reveals the highest TOC occurs at the middle part of the interval. With lake level rise, water column changes from saline to brackish, high primary productivity increases. And then, saline water column stratification and redox preservation condition in arid and hot climate are key controlling factors on the organic enrichment of the fifth shale interval in the Biyang depression. In the Lower part of Third-member shale interval of Shahejie Formation in the Zhanhua depression, the highest TOC occurs at the late EST and early HST. With the lake level rise from the EST to the HST, water column desalination and terrigenous supply enhanced, resulting in higher TOC content in the late EST and early HST.6. Formation models of organic-rich shale in the two typical rift basin are summarizedOrganic matter enrichment patterns are slightly different between the fifth shale interval of Hetaoyuan Formation in the Biyang depression and the lower part of Third-Member shale interval of Shahejie Formation in the Zhanhua depression. The organic matter enrichment of fifth shale interval of Hetaoyuan Formation in the Biyang depression has the following characteristics. The fifth shale interval is formed in the EST, The early EST is saline water environment with lake water column has high salinity and is suitable for preservation of organic matter. In the stage of LEST with the lake level rising, the water column is deepening and desalinating, and TOC content in EEST is better than that of LEST with the further increase of lake level, depositional environment changed from freshwater weak oxidation to oxidation with the very low organic matter content. The organic matter enrichment in the lower part of Third-Member shale interval of Shahejie Formation in the Zhanhua depression has the following characteristics, in the EEST is saline water environment, which is dominated by arid climate. The lake has high salinity and low water column is not suitable for organic matter preservation with low TOC content. In the stage of LEST and early EHST, the deepening of lake water, high paleo-productivity and stratified salt water are resulted in the highest TOC content, In the later LHST, the content of organic matter and TOC are decreased with the increase of terrigenous detritus. The third-order sequence represents shallow to semi-deep lake environment with lake water salinity from salt to fresh water, in general the organic matter content is high, and in the stage of LEST and EHST have the highest TOC content.