Research Of Prototype Basin And Paleogeography Vicissitude Of Mesozoic In San Jiang Area

The Sanjiang region limited by the Songhuajiang River, Heilongjiang River, Wusuli River is the object of study in this paper. Based on previous studies, combined with a lot of field work and Drilling and seismic data, a comprehensive systematic study have conducted to the group of Mesozoic basins in the area, which established a regional stratigraphic framework, analyzed the tectonic characteristics and evolution of the dynamic mechanism, conducted sedimentary research and calculation of erosion, reconstructed the palaeogeographic and restored the basin prototype. From the point of lithofacies palaeogeography it forecasted spatial-temporal distribution law of favorable source rock, reservoir and cap rock, predicted the resource potential of the basin and favorable exploration area.The distribution of residual stratum nowadays shows that the stratum of Sanjiang region in the early middle Triassic was missing; Late Triassic stratum are developed in Hulin Basin in the southern elongated strip distribution; Late Jurassic strata are mainly distributed in Suibin Depression, Sanjiang Basin and southern of Hulin Basin; Early Cretaceous strata basiclly cover the whole area; Late Cretaceous strata distributed in the Qianjin Depression of eastern Sanjiang Basin, Boli Basin, Jixi Basin and Ning'an Basin.Through depth study it is found that there was a large integrated prototype basin in Sanjiang region during Cretaceous, rather than the current independent residual basin. The Sanjiang region was a segmentation depression in Late Jurassic, developed marine-continental delta to littoral and shallow marine deposits in the area of Suibin Depressiong and Hulin Basin. During the period of Didao Formation to Chengzihe Formation deposition, Early Cretaceous, the Sanjiang area was gradually separated from the evolution of rift basins as a unified large inland lake, formed fluvial facies, (fan) delta facies, shore-shallow lake facies and semi-deep lake sedimentary system. In the period of Muling Formation deposition, the Sanjiang area was still a unified basin, with the (fan) delta facies, shore-shallow lake facies and semi-deep lake sedimentary system.Palaeogeographic studies have shown that there were many times Mesozoic transgression mainly from the east in this area; Zhangguangcai Mountain, Greater Higgnan Mountains and Wandashan Block were the main source areas in Mesozoic. During this period at least 5 significant ancient abrupt climate events occurred, that the dry Late Jurassic thermal event, the Early Cretaceous temperature and humid coal-event, the earliest Late Cretaceous hot events, early and middle Late Cretaceous warming wet events, and Late Cretaceous drought events.Since the Mesozoic, local mantle convection and back-arc spreading caused by Pacific Plate subducted towards Eurasia is the main dynamic mechanism. In the Early Cretaceous (Didao period) the basin was in the extension rift stage; Mid Early Cretaceous (period Chengzihe and Muling) in depression stage; From Later Early Cretaceous to Tertiary, Xingkai Terrane northward obduction formed a series of nearly EW thrust faults, so that unified basin destructed, resulting in the current distribution of residual basin.Through analysis of Mesozoic source rocks, reservoir and other reservoir conditions, oil and gas vision of the residual basin was evaluated, and Tangyuan Depression of Yishu Graben, Suibin Depression in Sanjiang Basin and Jixi basin were selected out as the next favorable exploration areas.