| Yitong Basin is a Cenozoic fault basin as the northward extension of Tan Lu Fault,located between Changchun City and Jilin City in the central of Jilin Province, shown witha NE long and narrow distribution and the area of approximately2,400km2. MoliqingFault Depression is a secondary tectonic unit of Yitong Graben. A lot of explorationresherch have been carried out previously, and Moliqing Oilfield has been found withexploration target layer of Shuangyang Formation.On the basis of previous conclusions, drilling, seismic and logging data were used tocarry out a research on Shuangyang Formation, Moliqing Fault Depression in this study.Moreover, the sedimentary environment of this sedimentary period was analyzed, theclassification of sand groups and single layers revealed by drilling were finely divided aswell as the sedimentary mode and microfacies distribution characteristics of subaqueousfan.Firstly, the division boundaries of sand groups were determined with the stratigraphiccyclicity as the basis, the key bed as the control and the electric logging curve as a means.Due to the longer hanging wall interval, multi-phase multiple fan bodies were developedvertically with a greater difference in sedimentary scale and more thin interbeds, leadingto a difficulty in establishing the division mode of standard well for promotion in thewhole region. Through repeated exploration, the division criteria of standard interval atdifferent depths and sections were determined, i.e., spatial complementation wasimplemented to achieve the division of sand group interface in the whole region. Thesedimentary rhythmicity of footwall strata in Member2of Shuangyang Formation wasrelatively clear, and therefore the division of standard well could be used to promote thismode for division of sand groups in the whole region.Secondly, single layers were divided within the boundaries of sand group, i.e., tocalibrate the accurate isochronous surface. The standard well was selected for dividingsingle layers to establish the standard profile, so as to build the backbone profile network. Consequently, the divided results of standard comparison profile were extended towardsthe comparison profile along the short axis. The method of adjacent well comparison withgradual spread was used to realize the unified closure of backbone profile and otherprofiles. The whole region was divided into hanging wall and footwall, where the hangingwall was divided into seven sand groups and53single layers, and the footwall wasdivided into six groups and24single layers. Influenced by faults and revealing degrees,the strata in Member2of Shuangyang Formation were truncated from west to eastgradually.Furthermore, the regional sedimentary background was analyzed. Combined withcore observation and the analysis of laboratory and logging data, facies and subfacies weredivided, and thus into microfacies. Through core observation, lithologic and faciessedimentary characteristics were summarized to establish a correspondence betweenvarious microfacies of subaqueous fan and logging curves. The sandstone contours graphswere used for controlling fan distribution range, and well-log facies identification wastaken as a means to comprehensively describe the fan body. The cores of eight wells inWell Y45of the study area were observed and described with total core length of318.7m.Then, various types of rock samples were found with visible slump structures to reflectunstable sedimentations, including muddy gravels and clods. The pebbled sandstone facieswere relatively developed. A systematic study was carried out on mudstone color,sedimentary structures, grain size distribution characteristics and other facies indicators.Finally, the Member2of Shuangyang Formation in the study area was identified as thesedimentary system of nearshore subaqueous fan. The overall sedimentary characteristicswere shown as the nondevelopment of inner fan with braided channels and relativelynondevelopment of lacustrine mudstone, while the mudstone developed with greatthickness and massive bedding of monolayer. The mudstones were mainly gray and black,indicating that the sedimentary environment was deepwater lacustrine reducingenvironment with common Bouma sequence and AAAA facies sequence, i.e.,high-density flow thick-layer glutenite sedimentation, or CDE facies sequence, i.e.,low-density flow siltstone transited to mudstone facies.In conclusion, the steps and achievements on fine division and comparison of singlelayers in the Member2of Shuangyang Formation, Moliqing Fault Depression wereanalyzed in this study. Compared with previous studies, a more rational division andcomparison scheme was obtained with clearer cyclicity. A huge difference existedbetween hanging wall and footwall in rhythmic characteristics, main waterway migration law, the size and number of fan bodies, basic electrical characteristics of reservoirs andwater-depth conditions. However, there was no simple superposition relation, but a certaindistance of strike-slip existed, preliminarily determined as8km or more. Meanwhile,detailed research was carried out on the sedimentation and distribution characteristics ofnearshore subaqueous fan in this region. |
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