| As an important part of petroliferous basin analysis,studying depositional filling evolution is of great significance in（1）reconstructing the paleogeographic pattern and reproducing the evolution process of sedimentary basins and（2）determining the source-reservoir-cap combination and predicting the petroleum distribution in the basin.Depositional filling of lacustrine rift basins is more sensitive to changes in tectonic activities,climate,provenance,and depositional base level,compared to those marine basins on stable continental margins.Therefore,the stratigraphic structure,sediment filling characteristics,depositional system configuration,sedimentary environment and evolution of the continental rift basin tend to be more complicated and changeable.In order to accurately explore the petroleum resources in these basins,it is essential to systematically study the evolution of basin fill under the influence of multi-geological factors.Located in the northeastern margin of the Qinghai-Tibet Plateau and west of the Hexi Corridor basin group,the Jiuquan Basin is one of the most important petroliferous basins in northwestern China.The basin was developed on the Paleozoic basement and was a superimposed basin composed of the Early Cretaceous extensional rifting basin and Cenozoic compressional depression basin.Affected by the collision of India-Eurasian plate since the Cenozoic era and the extrusion uplift of the Qinghai-Tibet Plateau,the Jiuquan Basin formed abundant structural reservoirs in the thrust zone adjacent to the Qilian Shan orogenic belt,becoming the key target of hydrocarbon exploration over the years.However,after decades of exploration and development,the potential of such reservoirs has been diminishing and the exploration difficulty has been gradually increasing.Looking for new oil and gas reservoirs has become a problem to be solved urgently in the basin for maintaining a stable production.The Qingnan Subsag is located in the west of Jiuquan Basin and is a typical small petroliferous subsag.It is also a key area for achieving petroleum production increase in Jiuquan Basin.Evaluation of petroleum resources shows that the abundance of hydrocarbon resources in the Qingnan Subsag ranks the fifth in the country,with a proven rate of 39%.However,the proven rate of similar sags has reached 60%,indicating that the potential for increasing reserves in the Qingnan Subsag is still large.The reason why the Qingnan Subsag is rich in petroleum resources lies in the thick and high-quality source rocks developed during the Early Cretaceous rifting.With the highly effective source rocks,the search for reservoirs has become the direction and focus for more petroleum discovery in this region.At present,the Lower Cretaceous Xiagou Formation is a key exploration layer in the Qingnan Subsag,and is also considered to be a key interval for oil and gas breakthroughs.However,the sedimentary characteristics,depositional processes,temporal and spatial distribution,controlling factors,vertical evolution,depositional models,and reservoir distribution of the Xiagou formation currently still lack systematic understanding,which directly restricts the further hydrocarbon exploration in this area.In order to solve the current exploration difficulties,this paper makes full use of various geological data including cores,well logs and lithology,geochemistry,paleontology data and three-dimensional seismic reflection data covering the whole study area to systematically study the basin filling evolution of Xiagou Formation within the isochronous sequence stratigraphic framework.Firstly,the depositional setting characteristics of the study area are analyzed in detail,including tectonic activities,basin subsidence,syndepositional palaeogeomorphology,provenance system and climatic conditions to rebuild the depositional environment.Secondly,within the framework of sequence stratigraphy,the depositional system characteristics and basin filling pattern of Xiagou Formation are analyzed in detail.Combined with the depositional setting characteristics,the controlling factors of basin fill in the study area are discussed,and the vertical evolution model of basin fill is constructed.On the basis of this,aiming at the reservoir rocks in the study area,their sedimentary process and evolution laws are clarified and the sedimentary model is established.Finally,the direction of further petroleum exploration in the study area is suggested.Chapter 1 introduces the project support,the objective and significance of this study,summarizes the research status and frontier of lacustrine rift fill,fan delta system and also puts forward some shortage in these academic researches.For example:（1）the previous experts studied sediment fill in lacustrine rift basins and proposed depositional evolution models.Although these models provide good guidance for the distribution and prediction of the sand bodies in the basin,they have little application for understanding the sedimentary characteristics and processes of deep lacustrine mudstones.It is indicated that there are differences in mineral composition,sedimentary structure and organic matter enrichment of deep lacustrine mudstones.These differences have a genetic relationship with the depositional environment and filling process in the basin.Therefore,it is necessary to strengthen the study on the relationship between basin fill and mudstone deposition in the basin.（2）As another example,although previous studies on the fan delta system are extensive,including facies architecture,depositional processes,sequence patterns,and controlling factors,and have built fan delta depositional models,the main focus of these studies is on fan delta systems occurred alongside basin margins,where their sediment supplies come from sources outside the basin.However,there are relatively few systematic studies on the fan delta alongside intrabasinal structural highs.The source of this type of fan delta comes from the interior of the basin,that is,the erosion of structural highs.Development characteristics of this fan delta are closely related to the evolution of intrabasinal structural highs.For example,in a rift basin,as the lake level falls,the exposed area of the structural high will increase,which is expected to provide more sediments and produce larger fan delta.As the lake level rises,the exposed area will be reduced or even the structural high will be submerged by the lake level,making a smaller fan delta or even the non-development of fan delta.However,the lake level change has a relatively small impact on the sediment supply of extrabasinal provenance for the associated basin margin fan deltas.Therefore,the depositional model of fan deltas at the margin of the basin may not be consistent with the fan delta along intrabasinal structural highs.This chapter also determines the contents of this thesis on the basis of the exploration status and existing problems of the Qingnan Subsag,and the research approach of this thesis based on the availability of various subsurface geological data.This research helps to deal with the current issue of the study area and thus has practical significance for the hydrocarbon exploration.Additionally,some of the research results can also make up for the deficiencies of the above research aspects,which has certain academic value.Chapter 2 mainly introduces the geological settings of Jiuquan Basin and Qingnan Subsag respectively.Jiuquan Basin is located at the intersection of Tarim block,Alashan block,and the Qilian Shan Caledonian fold belt.The basin has undergone three major tectonic stages:basement formation stage,Early Cretaceous rift basin stage,and foreland compressional depression basin stage since the Eocene.The development of the Early Cretaceous rift basin is mainly caused by the transtensional stress caused by the collision between the Lhasa block and Asian plate.A series of NNE graben and semi-graben structures have been formed in the basin,composing the secondary tectonic units（sags）within the basin.The tectonic evolution of the rifted basin mainly experienced the initial rapid rifting period,the rifting extension period and the post-rifting period.During the Late Cretaceous,influenced by the late Yanshan movement,uplift and erosion occurred in the basin,the lake diminished,and the evolution of rift lakes was terminated.The Cenozoic basin tectonic evolution was remote-controlled by collision between the Indian plate and the Eurasian plate and was further divided into the compressional depression period since the Eocene-Miocene and the thrust-fold period since the Pliocene.Overall,the Jiuquan Basin shows a double-layer structure.The sedimentary cover layer in the Jiuquan Basin mainly consists of Lower Cretaceous,Cenozoic Paleogene,Neogene and Quaternary strata.The Qingnan Subsag is located in the Qingxi Sag in western Jiuquan Basin.The evolution of the Qingnan Subsag and the Jiuquan Basin was characterized by isochronism and synchronicity,and also experienced the whole process from the early extension to the late compression.During the Early Cretaceous,Qingnan Subsag presented a single half-graben structure with faulting in the east and overlapping in the west.Development of the Chijinpu Formation,Xiagou Formation,and Zhonggou Formation correspond to the initial rifting period,rifting extension period,and post-rifting period,respectively.The hydrocarbon accumulation in the study area is closely related to the evolution of the later tectonic evolution.The intense subsidence caused by compression in the Cenozoic made the Lower Cretaceous hydrocarbon source rock deeply buried and matured.Meanwhile,many structural traps were produced by structural inversions,and fractures generated by compression were also conducive to the reforming of reservoirs and the hydrocarbon migration.Chapter 3 describes the construction of the Lower Cretaceous isochronous sequence stratigraphic framework of the Qingnan Subsag through the recognition of the sequence boundaries based on the data from outcrops,drill wells,and 3D seismic cube.The sequence boundaries are identified by the ancient weathering surfaces,scour erosion surfaces and abrupt lithofacies change surfaces on outcrops,the large contrasts in well lithology and large pattern and amplitude contrasts in wireline logs,the incongruous stratigraphic relationship reflected by onlap,downlap,and truncation reflections on seismic sections.The sequence boundaries recognized by drill well and seismic data were got corroborate mutually,and then were traced and closed on the whole seismic cube of the study area.The Lower Cretaceous rift filling succession of the Qingnan Subsag was divided into three second-order sequences（Composite sequences）:CSQK1c（Chijinpu Formation）,CSQK1g（Xiagou Formation）,and CSQK1z（Zhonggou Formation）.As the study interval,the entire Xiagou Formation was further subdivided into three third-order sequence:SQK1g0,SQK1g1,and SQK1g2+3 in ascending order.Chapter 4 analyzes the depositional setting of the Lower Cretaceous Xiagou Formation in the Qingnan Subsag,including the characteristics of the basin subsidence and tectonic activity,the paleogeomorphology,the configuration of the provenance systems and the paleoclimate conditions.By using equilibrium profile and quantitative back-stripping technique in the grid line sections of the study area,the subsidence of the basin in each of the three sequences of the Xiagou Formation is calculated,and the basin subsidence rate is obtained in combination with the duration time of each sequence development.At the same time,using the fault activity analysis method,the boundary fault activity rate is calculated.From SQK1g0 to SQK1g2+3,the boundary fault activity gradually weakened,and the study area experienced a transition from the early border-fault dominated differential subsidence to the late nearly uniform subsidence.Petrel modeling software was used to restore the original topography during the development of the three sequences of the Xiagou Formation based on the basin subsidence data.As a whole,the study area was characterized by a geomorphological landscape with deep in the east and shallow in the west,and the topographic gradient of the hanging wall has become less steep from the early stage to the late stage.Based on the analysis of subsidence and the restoration of tectonic paleogeomorphology,the source and sink areas were evaluated macroscopically.The contour map of the rate of coarse-grained sediments was used to delimit the source branch and the seismic reflection feature was used to determine the progradation direction of sediments,and combined with the seismic inversion of seismic impedance data constrained by well logs,this paper characterized the scope of source,and determined the provenance systems of Xiagou Formation in Qingnan Subsag.Finally,the three major source areas of the Xiagou Formation in the study area are identified:the Southern Uplift source area,the Yabei Uplift source area and the Qingxi Structural High source area.Based on the records of pollen fossils and carbon and oxygen isotopes of Xiagou Formation in the study area and its peripheral area,the climatic conditions of the Xiagou Formation in the study area are indicated.It shows that the climate changed from an early warm and humid climate to a semi-arid climate in the middle and late stages.Chapter 5 systematically studies the basin filling characteristics based on a large amount of core data,well log and lithology data,and seismic data,using the research approach of the integration of"point（wells）-line（cross-sections）-plane（planforms）",and finally reveals the sediment filling pattern of the Xiagou Formation in the Qingnan Subsag.The research area mainly develops the fan delta,shallow and deep lacustrine systems.Based on the structural location of the fan delta,it is subdivided into the structural high margin fan delta and the boundary fault margin fan delta.The structural high margin fan delta consists of subaerial facies and subaqueous facies.Both facies were dominated by glutenite and conglomerate with a variety of bed types.Sediments in this fan delta were mainly deposited through gravity flows,including coarse-grained debris flows,turbidity current flows,and slumps.The boundary fault margin fan delta is supplied by provenances from the outside of the Qingxi Sag,which are the southern Uplift and the Yabei Uplift.Most or the entirety of the fan delta was submerged in water and often deposited within deep lacustrine mudstones.The sediments are mainly massive coarse-grained conglomerate with a single bed type,and were dominated by coarse-grained debris flows.The deep lacustrine system mainly develops black mudstone and dolomitic mudstone with massive bedding and horizontal bedding.Laterally,the facies belt of the structural high margin is wider,and its extension direction is the same as the dip of the hanging wall,and the dispersal direction of the boundary fault margin fan delta is consistent with the strike of the fault.Vertically,from the SQK1g0 to SQK1g2+3,the area of the fan-delta system gradually decreased and the lake range gradually expanded,indicating that the Lower Xiagou Formation was characterized by a retrogradational basin filling pattern.Chapter 6 mainly discusses the controlling factors of depositional systems and basin fill,based on the analysis of the depositional setting,depositional systems,and basin filling characteristics,and establishes the basin filling and evolution model of the Xiagou Formation.The syndepositional paleogeomorphology has an important controlling effect on the fan deltas.First of all,it controlled of the fan delta sediment transport and deposition:（1）the boundary fault margin has a very steep slope（45?）,resulting in rapid deposition of the fan delta sediments mainly through coarse-grained debris flows,and（2）structural high margin slope is relatively gentle,sediments were deposited by debris flows,slumps,and turbidity flows;secondly,the effect is manifested in the control of the extension of fan deltas;the structural high margin fan delta mainly extended along the structural high margin slope into the basin,and the boundary fault margin fan delta extended along the fault trace.The subsidence of the hanging wall and the weakening of the boundary fault activity affect the sediment supply of the source area,and thus controlling the deposition and filling of the Qingnan Subsag.The continuous subsidence of the hanging wall caused the relief and area of the Qingxi Structural High gradually decreased,and thus the source supply capacity was continuously weakened,which makes the structural high margin fan delta gradually shrunk.The decrease of the boundary fault activity reduced the isostatic uplift of the footwall,which caused the decrease of the area of the fan delta on the boundary fault margin.Climate also has important control over sediment fill,which is mainly reflected in climate conditions affecting rainfall and thus affecting the weathering and erosion rate of the parent rock in the source area and also the efficiency of sediment transport.In the early warm and humid conditions,the larger volume and frequency of rainfall fastened the source erosion rate,and a large amount of precipitation can effectively transport the source sediments into the basin;In the middle and late semi-arid conditions,the volume and frequency of rainfall decreased,reducing the erosion rate of the source area and also lowering the transport efficiency and thus decreasing the sediment supply.In general,the basin fill of the Xiagou Formation in the Qingnan Subsag was controlled by multiple geological factors.Chapter 7 discusses the types of reservoir rocks in the Xiagou Formation,that is,fan delta glutenite and dark laminar dolomitic mudstone with fractures,and reveals the sedimentary processes and evolution of structural high margin fan deltas and deep lacustrine mudstones based on the full understanding of the depositional setting,basin-filling characteristics,and controlling factors and mechanisms,and clarifies the distribution of the reservoir rocks to guide the future hydrocarbon exploration of the Xiagou Formation in the Qingnan Subsag.From SQK1g0 to SQK1g2+3,mainly influenced by basin subsidence and climatic conditions,the structural high margin fan delta,has undergone a depositional evolution process from the most developed to terminated,which characterizes a retrogradational stacking pattern.Based on the sedimentary characteristics and evolution laws of the fan delta,the specific characteristics of intrabasinal structural high margin fan deltas are further summarized:（1）the fan delta is dominated by gravity flow deposition,because structural highs are often isolated in lake basins.Perennial rivers or streams are unlikely to develop at this position.Therefore,the sediments are mainly transported and deposited during heavy rains,resulting in discontinuity in deposition.（2）Within individual sequences,the fan delta presents a retrogradational accumulation pattern because of the decrease of the exposed area of the structural high and the decrease of sediment supply due to the rise of lake level from lowstand to highstand;（3）the development of Fan delta terminates once the structural high is submerged;and（4）The fan delta often develops in the early period of the basin evolution,since the holistic subsidence of the basin in the late rifting period or fault-depression transition period causes the structure high to be covered.Through the observation and statistics of mudstone cores,the distribution rules of massive mudstone and laminar mudstone are clarified.The massive mudstone is mainly distributed in SQK1g0 and the laminar mudstone is mainly distributed in SQK1g2+3.During the development period of SQK1g0,the study area showed a typical half-graben structure,with a small lake basin and a large topographic gradient.Strong boundary fault activity and wet climatic conditions prone to trigger mountain floods and landslides,causing severe agitation to the lake’s water body,resulting in lake water mixed.During the process,very fine-grained sediments in the deep lake were rapidly accumulated,forming the massive mudstone.Due to the mixing effect of lake water and the dilution of fine sediments,the organic content of massive mudstone is relatively low.During the development of SQK1g2+3,the range of the lake was enlarged and the topographic gradient was relatively gentle.The weaker tectonic activity and semi-arid climatic conditions reduced the efficiency of extra-basin sediment transport and supply.The lake water is less affected by external factors,and is often prone to seasonal stratification,and thus triggers the formation of laminar mudstone,which is composed of organic matter-rich algae lamina and dolomite lamina.Finally,according to the depositional filling of glutenite and laminar mudstone in the entire Xiagou Formation,a possible future exploration strategy was proposed,namely that SQK1g0 is dominated by exploration of glutenite reservoirs,SQK1g1 is dominated by exploration of glutenite reservoirs and laminar dolomitic mudstone fractured reservoirs,SQK1g2+3 is dominated by exploration of laminar dolomitic mudstone fractured reservoirs.The innovation of this paper mainly includes:（1）Compared with the basin margin fan delta whose sediment is supplied by provenances outside the basin,the intrabasinal structural high margin fan delta which is supplied by intrabasinal provenances（i.e.,the structural high themselves）is relatively less studied.Intrabasinal structural highs are often the favorable locations for hydrocarbon migration and accumulation.Therefore,the fan deltas alongside the structural highs have important value of hydrocarbon exploration.In order to solve the key issue of current petroleum exploration in the Qingnan Subsag,this paper systematically studies the structural high margin fan deltas and also summarizes their specific characteristics including sedimentary characteristics,depositional sequence patterns,and the main likely occurred stages,which aids better understanding and evaluation of similar systems in other rift basins.（2）Studying basin sediment fill often provide important clues for the characteristics,distribution,and prediction of sand bodies in the basin,but it has less significance in understanding the sedimentation and characteristics of deep lacustrine mudstones.Deep lacustrine mudstone can act as source rock and also unconventional reservoir rock.The lack of understanding of its sedimentary characteristics poses a high risk for oil and gas exploration.By systematically studying the basin fill evolution of the basin,the depositional process of the deep lacustrine mudstone under different depositional filling conditions is revealed.Besides,the corresponding depositional models of deep lacustrine massive mudstone and horizontal lamination mudstone of small lacustrine rift basins are established.This not only provides guidance for the exploration of oil and gas in the study area,but also helps to understand the sedimentation of deep lacustrine mudstone in a similar depositional setting. |
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