Sequence Stratigraphy And Depositional Systems In A Small Continental Faulted Basin

As an important type of petroliferous basin in China,faulted basins have extremely abundant oil and gas resources and they have been the focus of many geological studies.However,previous studies of hydrocarbon exploration have primarily focused on relatively large-scale faulted basins,whereas deep-seated faulted basins with small scales,shallow water,proximal sediment provenance and high rates of deposition have been neglected,especially in China.The Songliao basin,located in northeastern China,is a large-scale Meso-Cenozoic continental basin where develop many faulted basins with different scales in the lower part.At present,the hydrocarbon exploration in these faulted basins has made some breakthroughs.Several large gas fields such as Xushen1,Wangjiatun,Changde,Zhaozhouxi,Qinjiatun,Yaoyingtai and Changli have been discovered.However,those breakthroughs are mainly made in large-scale faulted basins(>1000 km2)and in the Yingcheng Formation and strata above,and few have been made in the clastic rocks within and below the Shahezi Formation in small-scale faulted basins.But the newly drilled Well Songs9 H,located in the northern Xujiaweizi Faulted Depression(a small faulted zone),has obtained a daily gas production of 20×104 m3 from the Shahezi Formation,revealing that the Shahezi Formation in small-scale faulted basins also possess good conditions for hydrocarbon accumulation,which can be taken as the new resources for the deep-seated natural gas in the northern Songliao basin.Due to little attention has been paid in the oil and gas exploration in small-scale faulted basins,their geological researches such as the regularity of hydrocarbon enrichment is relatively weak,especially the sedimentary-filling characteristics and their control on hydrocarbon accumulation in small-scale faulted basins are not clear.These deficiencies have severely limited the hydrocarbon exploration in those basins.Therefore,in this paper,the shahezi formation in the northern Xujiaweiz Faulted Depression has been taken as an object to study the sequence stratigraphy,sedimentary systems and law of hydrocarbon accumulation through an extensive database that includes cores,conventional logs,imaging logs and 3-D seismic data.Moreover,depositional evolution analysis,the hydrocarbon exploration potential and favorable targets of a small-scale faulted basin are discussed in addition to analytical data,which will be of great significance as a reference for the exploration of similar basins.The flowing achievements and recognition are obtained by above research.(1)A new structural model of sequence stratigraphy has been proposed,which can be applied to all continental basins.The sequence boundary and sequence stratigarphic framework has been studied by using the new sequence model,and sequence stratigraphic architecture styles with different source systems and tectonic setting have been analyzed.In a change cycle of lake level,the lowest and the highest lake level within a variation trend are the easiest surfaces to identified,and they can divide a sequence into two system tracts,namely lake transgressive system tract(TST)and lake regressive system tract(RST).During the relatively lake level rising and falling stage,if the lake level changes is stable at the beginning,the sequence can be divided into four system tracts,namely early lake transgressive system tract(ETST),late lake transgressive system tract(LTST),early lake regressive system tract(ERST),late lake regressive system tract(LRST).According to the new sequence structural model,the sequence boundaries with different level are identified,and one 2nd order and four 3rd order sequences are recognized.The four sequences are Sq1,Sq2,Sq3 and Sq4 from bottom to top,the lower three 3rd order sequence can be divided into TST and RST,and the top Sq4 can be divided into ETST,LTST and RST due to relatively abundant data.Then the 3rd order sequence framework of the Shahezi Formation in the northern Xujiaweizi Faulted Depression has been established,as well as the sequence stratigraphic architecture styles with different source systems and tectonic setting in a small-scale faulted basin.(2)By the observation and description of core,combining with logging data,the sedimentary facies of the Shahezi Formation in the northern Xujiaweizi Faulted Depression has been recognized.Geological and logging responses of different sedimentary facies are analyzed.Rock types of the Shahezi Formation in the northern Xujiaweizi Faulted Depression are mainly composed of conglomerate,breccia,sandstone,mudstone,coal seam and volcanic deposits.Fan delta,braided delta,lacustrine facies,near-shore subaqueous fan and volcanic facies can be identified.As the results of melange accumulation,fan-delta deposits are characterized by thick-bedded breccias and conglomerates.In image log,coarse deposits without apparent gradation are illustrated as unclear yellow spots with different sizes within the dark high-conductivity matrix.The large-scale box-shaped curve observed in gamma and resistivity logs is not clear and serrated.Braided-delta deposits are characterized by thick-bedded sandstones and conglomerates with well-sorted and sub-rounded gravels.Luminous yellow outline of well-sorted gravels can be observed in image log and large-scale box-shaped curve can be observed in gamma and resistivity logs.Lacustrine deposits are mainly composed of mudstones and coal seams which appear as luminous band in image logs.The volcanic deposits show large variations in their gamma-ray values and appear as line linear model in image logs.Near-shore subaqueous fan deposits are characterized by conglomerate which is supported by argillaceous matrix with dark color,and they are found on the steep slope at the basin marigin.(3)By the analysis of seismic profiles,types of seismic facies have been recognized,and their sedimentary interpretations also have been summarized by the calibration of wells.The distribution of seismic facies within sequence stratigraphic framework is analyzed.According to the external and internal reflection configurations,contact relations and seismic amplitude,ten kinds of seismic facies including foreset pattern,chaotic pattern,subparallel pattern,sheet-drape pattern,abnormal strong reflection pattern and so on can be observed.Among them,chaotic pattern with sphenoid and moundy shape and foreset pattern with sphenoid and lentoid shape are typical seismic responses of fan-delta deposits.Subparallel pattern represents braided-delta and lacustrine deposits.Foreset pattern with tabular shape represents braided-delta deposits.Banded seismic reflection with strong amplitude is typical seismic response of volcanic deposits.Sheet-drape pattern represents lacustrine deposits.Chaotic pattern with pyramidal shape is the results of volcanic eruption.Then,the distribution of seismic facies within sequence stratigraphic framework is described by the observation of each line seismic profile.(4)Within the 3rd sequence framework,sedimentary facies of the Shahezi Formation have been studied by the comprehensive analysis of multiple factors,as well as the sedimentary evolution.The specialty of sedimentation in a small-scale faulted basin has been summarized by the comparison with large-scale faulted basin.The Shahezi Formation in the study area is supplied by multiple source systems from the west and east.During the early sedimentary stage of the Shahezi Formation,the northern Xujiaweizi Faulted Depression with small scale,proximal sediment provenance is dominated by fan-delta deposits and the lacustrine facies is less developed.With the continuous activity of the Xuxi fault,depositional environments on the gentle eastern slope were changed from fan delta to braided delta,while the steep western slope was still dominated by fan delta deposits,and the area of lacustrine deposits continuously expand with an increase in the size of the basin.Based on the above researches,the specialty of sedimentation in the small-scale faulted basin has been summarized by the comparison with third member of the Shahejie Formation in the Dongying Depression,Bohai Bay basin,and then the sedimentary filling model in small-scale faulted basin is established.(5)Based on the study of sedimentary evolution of the Shahezi Formation,combining with the newly explorative achievements and the development characteristics of source rock and reservoir,the favorable exploration targets of strata formed during the intensively faulting stage in a small-scale faulted basin are discussed.Analytical data show that the source rock within the Shahezi Formation in the study area has large thickness,high TOC content and maturity,and types of organic matter are mainly composed of type II kerogen which is good for generating gas.The occurrence of source rock in the Shahezi Formation has two types,namely sand-mud interbed and coal-mud interbed.The favorable source rocks usually develop in the deep-faulted belt when the basin reaches its maximum size,and they are appeared as coal-mud interbed.Coarse deposits with low porosity and permeability formed in fan and braided deltas can be favorable reservoirs.The reservoir porosity of the Shahezi Formation is usually less than 10% and the permeability is less than 1×10-3um2.The thick and successive sand-conglomerate reservoirs are usually occurred at the basin margin.The explorative achievements of the Shahezi Formation reveal that gas productive reservoirs mainly are sandstones and conglomerates of Sq4,especially the Sq4-RST,and those reservoirs are near or above the favorable lacustrine source rocks.Therefore,for promoting the hydrocarbon exploration in a small-scale faulted basin,it will be of great significance to assess the location and abundance of effective source rocks and search for thick reservoirs that have good contact relationships with well-developed source rock,and fracture technology is also needed to improve the reservoir quality and achieve a high gas production rate.