This study aims to explore the impacts of terrigenous inputs on the organic matter type of semi-deep- to deep-lacustrine shales. Focusing on shales in the 3rd sub-group of the 7th oil group of the Triassic Yanchang Formation (also referred to as the Chang 7³ oil sub-group) in the Ordos Basin, we conduct comprehensive petrological, sedimentological, and geochemical analyses on core samples from four wells located in the northeastern, northwestern, and southwestern parts of the lacustrine basin during the deposition of the Yanchang Formation. Accordingly, the impacts of terrigenous inputs on the organic matter type are delved into. The results indicate that the samples involved are all rich in organic matter, with maturities ranging from 0.91% to 1.03%, suggesting medium to low maturity. The organic matter comprises types Ⅱ₁ and Ⅱ₂ kerogens, with kerogen macerals dominated by sapropelinite, followed by vitrinite. Notably, maceral types differ greatly across different wells. Shales in the Chang 7³ oil sub-group comprise seven lithofacies types: layered shale, shale with discontinuous to continuous laminae, silt-bearing shale with massive-discontinuous laminae, silty shale with massive-discontinuous laminae, layered silty shale, laminated silty shale, and massive silty shale. The layered shale, among others, is the least affected by terrigenous inputs, whereas the last four lithofacies are strongly affected. Shales from well W336 are weakly influenced by terrigenous clastic inputs. Correspondingly, type Ⅱ₁ kerogen occurs in this well, with macerals dominated by sapropelinite (with a content of 64% ~ 83%). In contrast, shales from the other three wells are more strongly affected by terrigenous clastic inputs. In these wells, type Ⅱ₂ kerogen predominates, with vitrinite content ranging from 15% to 42%. Despite the strong influence of terrigenous inputs, the parent materials of organic matter in the semi-deep to deep lacustrine environment remain dominated by aquatic organisms, producing favorable organic matter types. However, in the anoxic bottom water, higher plant debris accompanied by terrigenous inputs such as quartz and feldspar coexist with algae, leading to the formation of mixed type Ⅱ₁-Ⅱ₂ kerogen and comparatively less favorable organic matter types.

The study on sand body connectivity and distribution patterns is of great significance for well emplacement and injection-production pattern analysis in the A oilfield of the Weixi'nan Sag currently at a pre-development stage. Based on the current drilling data, seismic data, and fault development characteristics, this study investigates the connectivity, geometric morphology, planar distribution, and vertical evolution of composite sand bodies (multi-stage superimposed channel sand bodies) within the fault block using seismic forward and inversion modeling. The El₃Ⅰ oil layer group in the third member of the Liushagang Formation is developed in the fan delta-front sub-facies, which mainly consists of subaqueous distributary channels. The thickness of single-stage subaqueous distributary channel sand bodies ranges from 2 to 6 m, and the width of composite channel sand bodies varies from 50 to 100 m. Under the long-term transgression background, the subaqueous distributary channels in the El₃Ⅰ oil layer group are relatively narrow, forming superimposed and continuous composite channel sand bodies through lateral migration and vertical stacking. The long-term base-level cycles control the width of subaqueous distributary channels, while the mid-term base-level cycles control the thickness of these channels. The subaqueous distributary channels developed during the late stage of mid-term base-level fall are thicker than those formed during the early stage. Accordingly, quantitative relationships between channel thickness and width are established for the early and late stages of mid-term base-level fall, to finely depict the evolution patterns of channel sand body geometry and stacking styles across different stages. These findings provide important guidance for accurately predicting the planar distribution and channel width of composite subaqueous distributary channels at different stages of the mid-term base-level cycles.