The Heterogeneity And Controlling Factors Of Organic-rich Shale In He-third Member Of Eocene Hetaoyuan Formation, Biyang Depression

Shale oil and gas exploration and development in North American is great successful, triggering petroleum geological theory and technological innovation, and promoting the organic-rich shales research make important breakthroughs. With respect to organic-rich marine shale in the North American, shales in rift basins of Eastern China show significant heterogeneity of mineral composition, structure, organic carbon content and so on, seriously affecting the shale oil and gas production and prediction of favorable area. Recently, shale oil and gas research in the Biyang Depression focus on resources and reservoir evaluation, few detailed studies are taken into consideration on the controlling factors that lead to heterogeneity of shales. Meanwhile, the difficulty of drilling core specimen and field observations and the apparent homogeneity of shales hamper the comprehensive research of heterogeneity.In this study, according to the analysis of seismic, geological, core, well logging and oil testing data, and sedimentary petrology, sequence stratigraphy, seismic sedimentology theory and technical methods, the heterogeneity and controlling factors of the organic-matter-rich shales of Eocene Hetaoyuan Formation in the Biyang Depression is discussed. The study consists of (1) documenting the distribution of organic-rich shales and variation in organic carbon content (TOC) content, (2) reconstructing the depositional environment of organic matter, (3) identifying the OM accumulation mechanism through integrating the analysis of organic and inorganic geochemistry proxies of productivity, preservation and dilution within the sequence stratigraphic framework, and (4) exploring different approaches to indentify the heterogeneity of shales. The main results obtained are as follows:1. The He-third Member of Eocene Hetaoyuan Formation (Eh3) in the Biyang Depression can be divided into eight layers on the basis of seismic data, logging and cores. These layers correspond to four 3rd order sequences from bottom to top:Eh38 to Eh36 (Sequence Ⅰ), Eh35 and Eh34 (Sequence Ⅱ), Eh33 (Sequence Ⅲ), Eh32 and Eh3’(Sequence Ⅳ). Each sequence comprises lowstand system tract (LST), expanding system tract (EST) and highstand system tract (HST). Six organic-matter-rich intervals (ORIs) can be divided by the standards of TOC content larger than 2%, the thickness of single layer, continuous layers and interlayers larger than 10m and 20m, less than 3m, respectively, and are developed in EST of Sequence Ⅲ and Ⅳ, and lower HST. Of them, ORI 5 and 3 are believed to be the favorable ones in this depression because of large thickness, very wide distribution, moderate thermal maturity and high TOC content.2. According to the obversation of core and thin section, lamellar and massive structures are identified in study area, and the lamellar comprise four distinct types:organic-rich clay minerals laminae, carbonate-rich laminae, pyrite-rich laminae and silt laminae. The mineral composition include detrital minerals (quartz, feldspar), clay and carbonate, and clay and quartz are the mainly mineral, calcite, dolomite and plagioclase come second, and the contents of feldspar, pyrite and siderite are rarely. The vertical lithology combination distribute regularly in the sequence stratigraphic framework. Calcareous shales and dolomitic shales usually occurs in the early period of EST, the combination of clay-rich shales and calcareous shales is mainly developed in the middle of EST, and the combination of clay-rich shales, siltstone and fine sandstone occurs in the late period of EST. ORI 5 and 3 are mainly enriched in the middle and lower of expanding system tract, and near the maximum flooding surface, respectively. Two recurring patterns of TOC can be observed:a slight change; stacked units of sharp increase near bases and decrease upward. The TOC content varied widely and frequently in periphery of deep depression due to dilution of organic matter by turbidity flow, and had a slight change in the center of the deepest sag. In parasequence, with the lake lavel decrease, the TOC and free hydrocarbon content show no significant change in the center of the deepest sag but sharp increase near their bases and decrease upward.3. The TOC in shales from five wells are calculated using improved AlogR, CARBOLOG methods and multivariate regression analysis. The calculated values are compared with limited core analysis data, and the optimum method with high precision and wide applicability for the study area is selected. The results show that the prediction accuracy and universality of multivariate regression model are superior to that of the improved AlogR and CARBOLOG methods. On the one hand, the method of multivariate regression analysis could optimize well logs which response well to TOC, increasing the available logging parameters; On the other hand, it can avoid some well logs which are affected by special minerals, such as the pyrite. While for drilling wells without measured TOC data, TOC calculating models of seven blocks were established, which were divided based on the sedimentary facies or subfacies analysis. The multi-parameter regression analysis is optimum to predict the mineral content. Firstly, The logging parameters values corresponding to the sample depth is the average value of the vertical depth of 20cm, then analysis the multiple linear regression between logging values and measured mineral contents by the use of SPSS software, check the significance of regression equation and regression coefficients, and delete parameter variable, which do not reach significance level, and the forecasting models of contents of carbonate (calcite or dolomite), clay mineral and detrital mineral are established.4. The TOC content of ORI 5 and 3 distribute around the lake and gradually increase from the periphery to the center of lake. The TOC contents vary between 2.04 and 4.68%(avg.2.79%) in ORI 5 and range from 2.04 to 3.52%(avg.2.69%) in ORI 3. The ORI 5 shows higher TOC content with widely distributed. Combined with methods of seismic attribute technology, Geostatistical Inversion and geology, the distribution characteristics of different lithological combination are analysed. The distribution and vertical evolution of different lithological facies can be identified qualitatively by seismic attribute technology and identified quantitatively by geostatistical inversion analysis. The analysis of geophysical features of lithology indicates that shales, sandstone and calcareous and dolomitic shale, argillaceous dolomite can be separated by wave impedance. Then the present work addresses the geostatistical inversion and geology for discussion of the distribution of lithological facies of ORI 5. The results show that mudstone and shale are mainly developed in the north, calcareous and dolomitic shale usually occur in the central and southeast, and siltstone and fine sandstone interbedded with mudstone and shale are mainly developed in the periphery of the study area. According to the analysis of the relationship between lithological facies and seismic attributes for single well and cross well, it can be suggested that there is a higher and a lower form of Root-Mean-Square Amplitude (RMS) for the sand bodies of deltaic front. The thick channel sand of proximal of deltaic front are develop in southwest and northeast of study area with lower RMS, and the mouth bar sands at the delta front extends as a strip of northwest with higher RMS. Thin turbidite sands are deposited in predelta-deep lake. The RMS of predelta-semi deep lake is lower, corresponding to mudstone and shale. Dolomite are mainly developed in deep lake, located in the southeast of basin margin.5. The sedimentary environment of ORIs has been investigated based on the analysis of major, minor and trace elements, and the biomarker parameters of shales from ORI 5 and 3. The ORI 5 formed in relatively arid and saline environments, in which hot-arid changed to semiarid climate, saline to brackish lake, and phytoplankton to terrigenous higher plants upwards. ORI 3 was deposited in brackish lake under humid/warm climate with relative abundances of phytoplankton and algae.(1) The equation Boron (Bequ) content ranges from 272.14 to 968.98μg/g in all but one of ORI 5 samples, decreasing upward with significant change in the lower and slight variation in the upper; The B/Ga ratios of all samples generally exceed 4.5. These results indicate saline to brackish lake dominated during ORI 5 deposition. Sr contents decrease upward generally and vary between 294 and 4875μg/g (avg.724μg/g) in samples from ORI 5 with high Sr/Cu ratios (avg.19.26), reflecting hot-arid to humid/warm climatic conditions of deposition for ORI 5. The lower sterane/hopane ratios for ORI 5, combined with high C19/C23 TT and C24/C26 TT, suggest a significant contribution of terrigenous organic matter and heavy bacterial degradation.(2) The Bequ content varies between 349.81 and 474.31 μg/g in ORI 3 samples, and The B/Ga ratios generally exceed 4.5, indicating brackish lake occurred during ORI 3 deposition. Sr contents range from 352 to 696μg/g (avg.477μg/g), and Sr/Cu ratios vary between 5.3 and 15 (avg.9.4), reflecting humid/warm climate dominated during deposition of ORI 3.In addition, samples from ORI 5 and 3 show most of V/Cr ratios are greater than 2.0 with average of 2.09, Pr/Ph ratios are less than 0.6, and the value of Ce anomaly index is larger than-0.1. These proxies indicate relatively reducing environment in ORI 5 and 3. The differentiation of ΣLREE and ΣHREE is obvious, displaying low REE enrichments and high REE loss, the ratio of (La/Yb)N is obvious larger than 1, and the value of δCe range from 0.95 to 1.24 with an average of 0.99, which have unobvious or positive Ce anomaly. The value of (La/Yb)n and δCe reflects that the depositional rate of sediments was relatively low. The relatively higher sterane/hopane ratios for ORI 3 are in accordance with low C19/C23 TT and C24/C26 TT, reflect a predominance of algal organic matter.6. Although ORI 5 and 3 were developed in EST, the vertical distribution of TOC contents in each ORIs show obvious discrepancy. ORI 5 and 3 are mainly enriched in the middle and lower of expanding system tract, and near the maximum flooding surface, respectively, suggesting relative lake-level alone could not control the OM accumulation. Reducing conditions play a significant role in triggering OM accumulation of ORI 5 and 3, demonstrated by positive correlation between TOC and V/Cr and Pr/Ph values. High productivity may also account for the ORI 3, supported by the fact that Cu and Ni contents correlate positively with TOC values. These evidences indicate that favorable preservation deduced from stratified water column was the major factor controlling the OM accumulation of ORI 5, and high productivity coupled with oxygen-depleted bottom water in the relatively deep basin contribute to the OM accumulation of ORI 3.(1) The stratified water column with high salinity and anoxic conditions under arid climate enhanced OM storage in ORI 5. During early periods of EST for ORI 5 deposition, intensive evaporation under hot-arid climate resulted in saline lake, favoring stratified water column. The stratified water column and anoxic condition favor OM preservation, even in shallow water setting. With the rising of lake-level, fresh water inflow inside and decreased water salinity resulted in decreasing OM preservation.(2) The conjunction of high productivity following phytoplankton bloom coupled with oxygen-depleted bottom water in the relatively deep basin are favorable to OM preservation and accumulation in ORI 3. During ORI 3 deposition, high water inflow under wet climate results in brackish, deep lake with unstable stratification. The lower relative lake-level are not conducive to OM preservation during early EST periods. Abundant nutrients and dissolved inorganic carbon supply led to the bloom of algae. Under such conditions moderate to high primary productivity could be maintained. Meanwhile, higher lake level contributes to the formation of oxygen depletion bottom-water in deep basin, favorable for OM preservation.Additionally, slump turbidite fans were developed in lacustrine mudstones, which may influence the degree of preservation and enhance dilution of organic matter, resulting in the degradation of organic matter and decreasing of TOC values. The TOC profiles show large absolute changes in the periphery of deep depression and subtle increase and decrease in basin center locations.7. Three favorable organic-rich shale zones are predicted by the analysis of the thickness of ORIs, the distribution of TOC and lithological facies, sedimentary facies and interbeds of sandstone and mudstone.