Early Cambrian syndepositional structure of the northern Tarim Basin and a discussion of Cambrian subsalt and deep exploration

Using field geological survey,drilling and seismic data,combined with the study of regional tectonic evolution and structural deformation,as well as lithological and sedimentary analysis,we reconstructed the basin filling process and paleo-geography of north Tarim Basin in Early Cambrian,aiming to analyze the factors controlling the distribution and spatial architecture of the subsalt reservoir and source units and to define the favorable exploration direction.The Late Sinian tectonic activities in the northern Tarim Basin were characterized by different patterns in different areas,which controlled the sedimentary pattern in the Early Cambrian.The boundary faults of Nanhuaian rift basin in the south slope of Tabei uplift and the north slope of Tazhong uplift became reactivated in the Early Cambrian,forming two NEE and EW striking subsidence centers and depocenters,where the predicted thickness of the Yurtusi Formation could reach 250 meters.In the Xiaoerbulake period,the weak rimmed platform was developed in the hanging wall of syndepositional fault.Whereas the Nanhuaian rift system in the Tadong and Manxi areas were uplifted and destroyed in the Late Sinian,and appeared as gently slope transiting toward the subsidence center in the Early Cambrian.The former had the sedimentary features of hybrid facies platform and the latter had the sedimentary features of ramp platform.The black shale of the Yurtus Formation in the footwall of syndepositional fault and the reef bank of Xiaoerbulake Formation platform margin in the hanging wall in Early Cambrian constitute a predicable source-reservoir combination.The activity intensity of syndepositional fault controlled the thickness of black shale and the scale of the reef bank.It is suggested carrying out high accuracy seismic exploration to determine the location of Early Cambrian syndepositional faults,on this basis,to search the reef bank of Xiaoerbulake Formation along the faults westward,and then drill risk exploration wells at sites where traps are shallow in buried depth.

Syndepositional Fault Control on Sandbody Distribution at Two Eocene Confluence Deltas in the Dongying Depression,Bohai Bay Basin(Eastern China)

The prediction of sandstone distribution is the main target for hydrocarbon exploration and petroleum reservoir engineering,especially in a rift basin with a complex geological setting.During the Middle Eocene,the Dongying and Yong’an deltas converged in the Dongying depression of the Bohai Bay Basin,eastern China.So far,studies on the control of the existing syndepositional faults on the deltaic sandbodies of the two confluence deltas have been largely neglected.In this contribution,the sedimentary characteristics and spatial distribution of the two deltas,converging in the Dongying depression,were determined.In addition,the morphodynamic controlling factors of the deposits of the two deltas were identified based on core examination,log analysis,and seismic data analysis.During delta progradation,the sandbodies of the initially separated deltas were affected by the development of syndepositional tensional faults,which eventually also influenced the distribution of the sandstone reservoirs.The results of this study can not only be applied to petroleum reservoir engineering but also provide important new information for the studies on delta sandbody all over the world.

Syndepositional Fault Controlling on Depositional Filling of the Third Member of Shahejie Formation in Beitang Sag

In the continental lake basin whose structures were extraordinarily active, tectonism is an important factor in controlling the sequence and the depositional filling of the basin. This article reports the assemble patterns of syndepositional fault in the third member of Shahejie （沙河街） Formation in Beitang （北塘） sag. The results show that the comb-shape fracture system and the fracture transformation zone were developed in Beitang sag. These assemble patterns obviously controlled the sand-body and spatial distribution of sedimentary system. However, the steep slope belt of fault terrace, the multistage slope belt and the low uplift gentle slope belt controlled the development of sequence styles. Analyses of the spatial-temporal relationship of the assemble pattern of syndepositional faults and the sedimentary system help predict the favorable exploration zone.

Tectonic evolution and its control over deposition in fault basins: A case study of the Western Sag of the Cenozoic Liaohe Depression, eastern China

The main petroliferous basins in eastern China are Cenozoic fault basins, most of which have experienced two-stage tectonic evolution, i.e., rifting subsidence in the Paleogene and post-rifting thermal subsidence in the Neogene-Quaternary. The episodic tectonic evolution and syndepositional faulting had significant influence on the fault basins in terms of accommodation space, deposition rate, and depositional facies zones. In this study, the tectonic deformation characteristics and the tectonic-depositional evolution of the Western Sag of the Cenozoic Liaohe Depression were investigated by comprehensive analysis of the available geological and geophysical data using the modern theory of tectonic geology and the balanced section technique. The tectonic deformation of the Cenozoic fault basin was characterized by superimposed faults and depression. In addition, there existed relatively independent but still related extensional tectonic systems and strike-slip tectonic systems. The tectonic evolution of the fault basin involved five stages, i.e., initial rifting stage （E2s4）, intense faulting stage （E2s3）, fault-depression transition stage （E3sl2）, differential uplifting stage （E3d）, and depression stage （N-Q）. According to the characteristics of tectonic development and evolution of the Western Sag, the depositional evolution in the Cenozoic fault basin was divided into two stages, i.e., multi-episodic rifting filling in the Paleogene and post-rifting filling in the Neogene-Quaternary. The former rifting stage was further subdivided into four episodes with different characteristics of depositional development. The episodic faulting controlled the filling process and filling pattern of the Cenozoic Western Sag as well as the development and spatial distribution of associated depositional systems, whereas the syndepositional faults that developed in multiple stages in various tectonic positions controlled the development of depositional systems and sand bodies in the Western Sag. That is, the fault terraces on steep slopes controlled the development of sand bodies, the fault terraces on gentle slopes controlled the development of low-stand fan bodies, and the fault terraces or fault troughs in the central basin controlled the development of fluxoturbidite bodies.

Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

The characteristics of petroliferous plays in subtle traps within a sequence stratigraphic framework in the Dongying Depression are investigated in this study.Sand bodies within lowstand systems tracts(LSTs)of sequences,comprising incisedchannel fills,sublacustrine fans,deltas in LSTs,controlled by syndepositional normal faults,and sand bodies within transgressive systems tracts(TSTs)to early highstand systems tracts(HSTs),consisting of beach bars,and turbidites,controlled by the prodelta slope,paleorelief,and syndepositional normal faults,are good subtle reservoirs.Mudstones and shale of deep lake subfacies in TSTs to early HSTs of sequences are source and cap rocks.Abnormal overpressure is the dominant dynamic factor for hydrocarbon migration from source rock to the subtle traps.Normal faults,sand bodies,and unconformities function as conduit systems.Sand bodies distributed in the abnormal overpressure source rocks within LSTs to early HSTs are petroliferous plays in lithologic traps.The petroliferous plays in stratigraphic traps are controlled by unconformities at margins of the Depression.