Occurrence of Laminated Dolomitic Exhalative Rocks in Continental Rift Basins of Northwest China

Objective Researchers have recently discovered that sublacustrine sedimentary exhalative mechanism associated with volcanism,is the principle way to form lacustrine exhalative rocks.These rocks differentiate themselves from normal sedimentary rocks in their specificpetrofabric and material composition.

A near-shore clastic-carbonate mixing mode in a continental rift basin(early Oligocene,eastern Shijiutuo Uplift,Bohai Bay Basin,China):sedimentology,reservoir characteristics and exploration practice

A comprehensive sedimentary and reservoir analysis was conducted based on seismic,well logging,core and relative test data,taking Members 1 and 2 of Shahejie Formation of the early Oligocene in the steep slope belt,eastern Shijiutuo Uplift(STU),Bohai Bay Basin(BBB)as a case.The study indicates that a near-shore mixed fan deposit formed in the study area and developed characteristics and pattern of a high-quality reservoir.The mixed clastic-carbonate rocks constitute Members 1 and 2 of Shahejie Formation which developed along the steep slope belt and is named as a near-shore mixed fan.The mixed fan of the study area is mainly composed of microfacies of proximal channel,mixed deposited channel,mixed clastic beach,mixed bioclastic(grain)beach,with vertical multi-stage superimposition feature,and basically a similar shape as modern near-shore fans.It constitutes a new depositional type developing in the steep slope belt of a characteristic and complex lacustrine rift basin in the study area.This mixed fan in the steep slope of eastern STU is controlled by comprehensive factors including tectonics,clastic material supply,climate,palaeogeomorphology and hydrodynamic conditions.The reservoir quality of Members 1 and 2 of Shahejie Formation of eastern STU is,however,actually controlled by the sedimentary environment and diagenesis processes.Coarse-grained mixed rocks of near-shore fans,rich in bioclastics,can form excellent reservoirs,characterized by resistance to compaction,easy to dissolution,little influenced by burial depth and high production of oil and gas,which enable them become key exploration targets of medium-deep strata of BBB.Analyses of high-quality reservoir,its controlling factors and the oil and gas exploration implications of the near-shore mixed fan developing in the study area give a deeper insight into discussions of the same type of mixed rocks of other lacustrine rift basins worldwide.

Tectonic characteristics and structural styles of a continental rifted basin:Revelation from deep seismic reflection profiles

The Fushan Depression is a half-graben rifted sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The Paleogene Liushagang sequence is the main hydrocarbon-bearing stratigraphic unit in the sub-basin. Using three-dimensional（3-D）seismic data and logging data over the sub-basin, we analyzed structural styles and sedimentary characteristics of the Liushagang sequence. Five types of structural styles were defined： ancient horst, traditional slope, flexure slope-break, faulted slope-break and multiple-stage faults slope, and interpretations for positions, background and development formations of each structural style were discussed. Structural framework across the sub-basin reveals that the most remarkable tectonic setting is represented by the central transfer zone（CTZ） which divides the sub-basin into two independent depressions, and two kinds of sequence architectures are summarized：（i） the western multi-stage faults slope;（ii） the eastern flexure slope break belt. Combined with regional stress field of the Fushan Depression, we got plane combinations of the faults, and finally built up plan distribution maps of structural system for main sequence. Also, we discussed the controlling factors mainly focused on subsidence history and background tectonic activities such as volcanic activity and earthquakes. The analysis of structural styles and tectonic evolution provides strong theoretical support for future prospecting in the Fushan subbasin and other similar rifted basins of the Beibuwan Basin in South China Sea.

Dynamic Analysis on Rifting Stage of Pearl River Mouth Basin through Analogue Modeling

The Pearl River Mouth basin （PRMB） is a marginal sedimentary basin of the South China Sea. It trends NE and is divided into three segments from west to east by two NW-trending faults. Changing dramatically in structures along and across strike, the PRMB is a good example to analyze main factors that might control the process of a continental rift basin＇s extension. Through five series of analogue experiments, we investigate the role of different factors, such as pre-existing discontinuities of crust, rheological profiles of lithosphere, kinematics of extension and presence of magmatic bodies and strong crustal portions （rigid massifs） on the development of basin＇s structures. After being compared with the architecture of the natural prototype, the results of the analogue models were compared with the architecture of the natural prototype and used to infer the role of the different factors controlling the formation and evolution of the PRMB. The main conclusions are as follows. （1） Affected by pre-Cenozoic structures, the PRMB was controlled by crosscut NE- and NW-trending initial faults, and the NW-trending Yitong＇ansha （--~l~） fault may be a through-going fault along dip and offset the NE-trending rift and faults, while the Enpingdong （和统暗沙） fault might exist only in the middle and south. （2） The NW-trending faults may orient WNW to be sinistrally transtensional under SE to nearly NS extension. （3） The thickness ratio of brittle over ductile crust in Baiyun （白云） sag is lessthan normal, suggesting an initially hot and weak lithosphere. （4） The magma must have taken part in the rifting process from early stage, it may occur initially upon or slightly south of the divergent boundary in the middle segment. The flow of magma toward rift boundary faults caused extra vertical subsidence above the initial magma reservoir without creating a large extensional fault. （5） The rigid massif contributed to the strain partition along and across basin strike.

Reservoir formation conditions and key technologies for exploration and development in Shengtuo oilfield in Bohai Bay Basin

The Shengtuo oilfield is the largest monolithic oilfield in the Bohai Bay Basin and even in the continental fault basins of Eastern China.Since discovered in 1963,the Shengtuo oilfield experiences the early exploration stage of rapid reserve discovery and production,the middle exploration stage of high and stable production and development,and the late exploration stage of theoretical and technological innovation for expanding the field of exploration,research on fine development technology of high-water-cut reservoir for enhanced oil recovery,and maintaining slow decline of oil production.Re-serves of this oilfield increase in each stage.By the end of 2018,after more than 50 years of exploration and development,the Shengtuo oilfeld has cumulative proven oil reserves of 0.512 x 10°t,cumulative oil production of 0.193 x 109t,and it keeps stable annual production of more than 150 x 105t in 2018.Abundant hydrocarbon resources,large anticline structure near to oil source,multiple types of reservoirs and traps,and favorable hydrocarbon migration channels provide favorable hydrocarbon accumulation conditions for the Shengtuo oilfield.Fine development techniques such water-flooding in the early development stage,fine reservoir description in the high water cut stage,stratigraphic subdivision and well pattem vector adjustment,tapping remaining oil in rhythmic layers,etc,as well as conti nuous progress in exploration technologies of glutenite and turbidite reservoirs around the main body of the oilfield in the high-level exploration stage,provide effective supports for continuous increase of reserves and long-term high and stable production of the Shengtuo oilfeld.