The Distribution Characteristics And Resources Potential Assessment Of Oil Shale In The Upper Cretaceous Of Songliao Basin

Songliao basin is a Cretaceous large-scale Mesozoic-Cenozoic terrestrial oil shale-bearing basinlocated in northeastern China, and the oil shale sequences are mainly developed in Qingshankou andNenjiang Formation. It is important to investigate the distribution characteristic and assess theresources potential of oil shale, due to the low exploration degree in the whole basin.Based on the outcrop section and core observation, combine with the microscopy andwell-logging data, the Qingshankou (K2qn) and first-second member of Nenjiang Formation (K2n1-2)mainly developed lacustrine and delta facies, further subdivided into4subfacies and11microfacies.According to core and outcrop data, combine with the sand-stratum ratio and distributioncharacteristic of dark mudstone, the paper recovers the sedimentary facies distribution in the targetstratum: during the K2qn1, the northern plunge zone, western slope zone and southwestern uplift zonehave high sand-stratum ratios, indicate the deltaic deposit, whereas the ratio gradually decreasetoward the basin-center, and the sedimentary environment transit from shallow lake to semi-deep todeep lake, particularly the semi-deep to deep lake in the central downwarp zone, northeastern upliftzone and southeastern uplift zone are the best place to the oil shale deposition. During the K2qn2-3, thedeltaic environment expands and the lake area reduces. During the K2n1, only in the northern plungezone and northern northeastern uplift zone develop large scale deltaic environment, whereas in thewestern slope zone, southeastern uplift zone and southern northeastern uplift zone are mainly shallowlake area. During the K2n2, the scale of semi-deep to deep lake area expands rapidly, forming the thickoil shale layer in the Nenjiang Formation.The oil shale in the upper Cretaceous of the Songliao basin is mainly grey-black, black-grey anddark grey, argillaceous texture, develop the horizontal and massive bedding, occasionally weakdeformation structure. The microscopy and X-ray analysis reveal that the mineral composition of oil shale is composed of clay minerals,quartz, feldspar, calcite, dolomite and pyrite. The geochemistrydata reflect the element abundance of oil shale in K2qn is more variable than K2n1-2, it may be relatedto the multi-provenance in K2qn. The well-logging data of oil shale show the character of “three highand one low”, thus high GR, high AC, high Rd/Rs and low Den response.The oil yield and total organic carbon (TOC) show a good correlation in the oil shale of theSongliao basin. Based on the lithology, TOC and oil yield analysis, from the respects of burial depth,thickness and oil yield, reveal the horizontal distribution of oil shale: in the K2qn1, the oil shalethickness and oil yield varied, with the maximum thickness in the central downwarp zone andnortheastern southeastern uplift zone and the high oil yield area is in the southern basin, during thisperiod the industrial oil shale (burial depth less than1000m) have limited distribution, mainlydeveloped in the northeastern uplift zone and southeastern uplift zone. During the K2n1-2, thethickness and oil yield of oil shale distributed evenly, the relatively thick oil shale deposit in thecentral downwarp zone, southeastern uplift zone and northeastern uplift zone, whereas the oil yield isstable, the high oil yield area is mainly in the northern, central-eastern and southern basin, the miningcondition is better during this period, the burial depth of oil shale is less than1000m except theperiphery of central downwarp zone.The paper employs the statistical analogy to evaluate the oil shale potential resources in theSongliao basin. In order to obtain a more accurate result, through the statistical analysis of large wells,the paper reveal the distribution of dark mudstone within the basin, and set the dark mudstone as theobject of study, count the oil shale occurrence abundance in the dark mudstone, exclude the effect ofsandstone. Based on the above, the Northern Changchunling, Southern ChangchunlingQianguo-Nongan and Shenjingzi with high exploration in the Songliao basin are selected as thecalibrated area, rely on DoubleFox software to calculate the oil shale potential resources (burial depth<1000m) in the K2qn and K2n1-2of the Songliao basin are307.18gigaton, and the potential resourcesof shallow oil shale (burial depth <500m) are116.57gigaton. Due to the improve of oil shaleresources exploration, improved method of resource evaluation and the innovation of technology, theresources potential assessment of oil shale of the Songliao basin in the paper are more accurate andreliable than before.