天文旋回驱动下的有机质富集机制OA

As a key driver for paleoclimate evolution,astronomical orbital parameters effectively modulate sedi-mentary-filling processes and differential organic-matter enrichment within continental lacustrine basins.Taking well DW7 penetrating the Chang 7 oil-reservoir set of the Triassic Yanchang Formation in the Xunyi area of the Ordos Basin as the research object,this study established a high-precision cyclostratigraphic framework and an organic-matter enrichment model.The workflow integrated natural-gamma-ray logging data with total-organic-car-bon(TOC)and elemental-geochemical datasets,and adopted multiple approaches including spectral analysis,eCOCO-based sedimentation-rate estimation,astronomical tuning and filtering analysis.The results revealed well-preserved Milankovitch cyclic signals within the Chang 7 oil-reservoir set of the Xunyi area.Astronomical tuning anchored to the most stable 405-kyr long-eccentricity cycle constrained the total depositional duration of the Chang 7 oil-reservoir set to approximately 1.29 Ma.Combined with high-precision age anchor points,the basal and top boundary ages of this set were determined as(241.56±0.22)Ma and(240.27±0.22)Ma,respectively.Sedimentation rates derived from the eCOCO method ranged from 1.09 to 13.60 cm/ka.Four long-eccentricity cycles(E1-E4)and fourteen short-eccentricity cycles(e1-e14)were vertically identified via Gaussian filte-ring,on which basis the cyclostratigraphic framework was constructed.Reconstruction of the paleo-depositional environment indicated that the Chang 7 oil-reservoir set was predominantly deposited under warm-humid,fresh-to-brackish,anoxic semi-deep-to-deep lacustrine conditions.Both depositional settings and organic-matter enrich-ment varied synchronously with eccentricity cycles.This demonstrated that the paleoenvironmental evolution and organic-matter accumulation of the Chang 7 oil-reservoir set were governed by the 405-kyr long-eccentricity cycle,with superimposed effects from tectonism,volcanic-hydrothermal activity,water-column stratification and gravity-flow events.During the long-eccentricity maxima,amplified seasonal contrasts intensified East-Asian monsoon precipitation,enhanced terrigenous-nutrient influx and strengthened water-column stratification,giving rise to an enrichment regime characterized by"high primary productivity+favorable preservation".By contrast,the long-eccentricity minima corresponded to an arid-cold climate that suppressed productivity and degraded preser-vation conditions.Meanwhile,frequent gravity-flow events occurred during such periods,lowering organic-matter accumulation through clastic dilution and perturbation of the anoxic bottom water.