Prediction of fine-grained sedimentary lithofacies distribution based on astronomical cycle isochronous lattice: a case study of Triassic Chang 7 member of Fuxian area, Ordos Basin

Predicting the thickness distribution of different types of continental lithofacies is a fundamental task for selecting and evaluating continental shale oil-rich zones. Research on oil-bearing conditions, reservoir characteristics, and mobility of different types of lithofacies plays an important role in target area selection and the deployment of horizontal well sections. Based on core observation and logging identification, spectrum analysis of logging data was carried out. Stable astronomical orbital time cycles were introduced to perform spatio-temporal tuning. A high-frequency sequence isochronous lattice for drilling well comparison was established and the planar thickness variation trends of different types of lithofacies in each sequence cycle were quantitatively calculated, providing insights into lithofacies distribution patterns. Research on the 7th member (Chang 7) of Triassic Yan-chang Formation in the Fuxian area of the southern Ordos Basin showed that the natural gamma logging curves contained several sets of astronomical cycle information, among which 6 complete stable 405 kyr long eccentricity astronomical cycles could be identified. Based on the observation of the whole core section of Chang 7 member in well R203 and logging facies characteristics analysis, a reasonable high-frequency isochronous lattice was established for well-to-well comparison. The results showed that mud shale and laminated shale mainly developed in the bottom cyclic strata from Chang 73 to Chang 72 sub-member, and the fine-grained sandstone and siltstone were mostly developed in strata of cycles Ⅳ to Ⅴ from the middle and upper parts of Chang 72 to the bottom of Chang 71. Lateral comparison between wells showed that the mud shale and laminated shale lithofacies in the early cycles were widely distributed. During cycle Ⅰ period, these facies were mainly distributed in the southwest of the study area, and during cycle Ⅱ period, they were distributed in the west and east-northeast, with thicker storage in the central area. The fine-grained sandstone thickness distribution in strata of cycles Ⅳ to Ⅴ was controlled by a northeast-southwest sedimentary system, with a planar distribution from the north-northeast to the south-southwest, further extending to the southwest along wells ZF 27 to ZF 32. Three types of source and reservoir combinations were formed in Chang 7 member. The combination of laminated shale and fine-grained sandstone lithofacies occurred from the middle and upper parts of Chang 73 and Chang 72 sub-members to the lower part of Chang 71 sub-member, primarily distributed in the central-north, north, and northeast parts of the Fuxian area. The shale lithofacies thickness was well developed in Chang 73 sub-member, mainly distributed in the northeast, east, and southwest regions of the Fuxian area.