Depositional Systems And Reservoir Characteristics Of The Lower Cretaceous In The Northern Fulongquan Faulted Depression, Southern Songliao Basin

Some achievements have been obtained after long terms of explorations within Fulongquan faulted depression of the Southern Songliao basin, however, there also be certain problems according to further developments.This paper takes the Lower Cretaceous of northern Fulongquan faulted depression as an research subject, uses sedimentology, seismic stratigraphy and petroleum geology, based on regional geological settings, exploration experiences and numerous seismic data, synthesizes theories and practices, systematically studied distributing feature of the sedimentary facies and system in this area. Thus, ascertain that sedimentary environment controls distribution of sedimentary facies and system, and it's also the geologic foundation that affects source-reservoir-cap capabilities. Moreover, divide the district sequence stratigraphic framework, categorized the reservoir types.1.Study methods of sedimentary systemBased on division and comparison of borehole stratum as well as demarcation, tracing and closing of seismic reflector, together with uniform stratum division and comparison of the whole area, aim at each objective layer, then distinguish, divide and describe sedimentary facies on the basis of field, logging and seismic profiles. On the whole, division of seismic sedimentary facies can match sub-facies. Then, analyze facies origin, ascertain and chart the sedimentary system, thus indicating the formation, scale and distribution of each sedimentary facies and system. On this basis, analyzed sequence stratigraphic framework and reservoir rock distribution, and discussed source-reservoir-cap assemblage and allocation.2.Sedimentary system types and sequence stratigraphic characteristicsIn the study area, sedimentary model mainly is continental clastic sediment, namely, a continental fluvial- lacustrine basin. Sedimentary facies mainly includes fluvial, fan-delta, delta, shallow-lake, semi-deep lake and underwater fan, and partially develops alluvial fan and lakeshore facies. On the basis of conception of sedimentary system, integrate thickness change analyses of each sequence, as well as progradational reflection configuration of seismic facies analyses and planar distribution characteristics of sedimentary facies in each period, thus, categorize the regional sedimentary system as following: alluvial- fan delta system, fan delta- underwater fan system, fluvial- delta system and delta system.Primary achievements and recognition are as following:Fulongquan faulted basin is a continental fluvial- lacustrine basin and sedimentary facies mainly include alluvial fan, fluvial, fan delta, delta, lakeshore, semi-deep lake and underwater fan. Primary sedimentary systems are alluvial- fan delta system, fan delta- underwater fan system, fluvial- delta system and delta system.Reservoir body is single and mainly includes (fan) delta front sand body, channel sand body and sheet sand, forming single type of petroleum accumulation.Fulongquan faute basin develops three first-order sequences, five seconed-order sequences and twelve third-order sequences. And there develops corresponding system tract in each sequence, however, only few is complete.3 . Source-reservoir-cap assemblage characteristics inside sequence stratigraphic frameworkIn Fulongquan faulted depression, source rocks formed mainly in the period of the second-order sequence B (SSB, Shahezi Form.) and C (SSC, Yingcheng Form.). Thickness of effective source rock is between 30 and 60 meters in S3 and is from 60 to 100 meters during S6. Located in the center of lake basin and turned to be source depression and hydrocarbon generation center.During the developing process of second-order sequence, there all formed large scale of sand bodies both in the low-order and high-order between SSA and SSD, and these sand bodies formed fine reservoirs which are mainly distributed in the SSC and SSD.For Fulongquan faulted basin, regional cap rocks are mudstone of Qingshankou Formation. While there are some other cap rocks in the study area, for example, mudstone of the 1st member of Quantou Formation is the cap rock of oil layers of Denglouku Formation, and mudstone of Denglouku Formation is the cap rock of oil & gas layers of Yingcheng Formation. Study on the second-order sequence between the 1st member of Quantou Formation and Shahezi Formation indicates space-time matching and genesis relationships of each sequence forming unit in the sequence stratigraphic framework. And it also develops source-reservoir-cap assemblages for petroleum significances.