Huajun Gan

Sedimentary characteristics and model of gravity flow depositional system for the first member of upper Miocene Huangliu Formation in Dongfang area, Yinggehai basin, northwestern South China Sea

The gravity flow deposit were mainly developed in the lowstand systems tract (LST) of the first member of Upper Miocene Huangliu Formation (Ehl1) in Dongfang area, Yinggehai Basin, has become a valuable target for gas exploration and production. The gravity flow sedimentary characteristics of lithofacies associations, sedimentary texture, seismic facies and logging facies were described in detail on the basis of integrated analysis of cores, logging and seismic data. The sedimentary microfacies types composed of neritic sandbar, continental shelf mud, main channel, bifurcated or cross-cutting distributary channel, overspill, and natural levee are revealed under the constraint of high resolution sequence stratigraphic framework in the Ehl1. The gravity flow deposit system in the LST is divided into three evolution stages corresponding to periods of three parasequence sets. The gravity flow deposit was induced in the early LST, expanded rapidly in the middle LST and decreased slightly in the late LST. But its developing scale decreased sharply in the transgression systems tract (TST) and finally vanished in the highstand systems tract (HST). This spatial evolution rule is constrained by the integrated function of sediments supply of the Vietnam Blue River in the LST, the development of local gradient change in sea floor (micro-topography, i.e., flexure slope break), and the fall in relative sea level. On the basics of the deep study of the coupling relationship among the three main control factors, the sedimentary model is established as an optimal component of “source-channel-sink” for shallow marine turbidite submarine fan.

Paleogene Tectonic Evolution Controls on Sequence Stratigraphic Patterns in the Central Part of Deepwater Area of Qiongdongnan Basin, Northern South China Sea

In active rift basins, tectonism is extremely important for sequence stratigraphic patterns, affecting both the sequence architecture and internal makeup. Sequence stratigraphic framework of a Paleogene rift succession in Qiongdongnan Basin, northern South China Sea, was built using seismic profiles, complemented by well logs and cores. One first-order and three second-order sequences were identified on the basis of basin-scale unconformities, and seven third-order sequences are defined by unconformities along the basin margins and correlative conformities within the central basin. Through unconformity analysis and backstripping procedure, the Paleogene synrift tectonic evolution of deepwater area of Qiongdongnan Basin was proved to be episodic, which can be divided into rifting stage-I, rifting stage-II and rifting stage-III. Episodic rifting resulted in the formation of various types of structural slope break belts, which controlled different architectures and internal makeup of sequences. This study enhances the understanding of the control of tectonic evolution on sequence stratigraphic patterns and establishes relevant patterns in a typical rift basin, and further proposes the favorable sandstone reservoirs developing in different sequence stratigraphic patterns, which will be pretty helpful for subtle pool exploration in deepwater area of petroliferous basins.

Control of anticline crest zone on depositional system and its geological significance for petroleum in Changshaling, Yinger sag, eastern Jiuquan Basin

Yinger (营尔) sag is the main petroleum generative sag in eastern Jiuquan (酒泉) Basin, and only the Changshaling (长沙岭) oil resource has been found after 60 years prospecting. Changshaling anticline crest zone was formed above the regional uplift, and was slightly affected by over thrust during Tertiary Period. The anticline crest was identified as a structural transposition zone. Based on analysis of seismic facies, logging facies and seismic inversion technique, it has been demonstrated that anticline crest zone conducts drainage entering basin and dominates sedimentary detritus further dispersing, and the anticline crest zone controls the distribution of sand bodies and the development of sedimentary system consequently. The sequence stratigraphic patterns of multi-step fault belt in Member 3 of Xiagou (下沟) Formation in Lower Cretaceous in the anticline crest zone is composed of the development of sedimentary facies in plan and distribution of multi-step fault belt sand bodies in spatial. This research investigates the sand-control models and sequence patterns, and finally a model of reservoirs in anticline crest zone is identified.

Paleogene tectonic evolution controls on sequence stratigraphic patterns in the Fushan sag, northern South China Sea

Tectonism is of extreme importance to sequence stratigraphic patterns in continental sedimentary basins, affecting both the architectures and internal makeup of sequences. Sequence stratigraphic framework of the Paleogene system in the Fushan sag, northern South China Sea, was built using 3D and 2D seismic data, complemented by drilling cores and well logs data. One first-order, three second-order and seven third-order sequences were identified. Analysis of paleotectonic stress field, unconformities and subsidence history showed that the Paleogene tectonic evolution presented significant characteristics of multistage and episode, and can be divided into three stages: rifting stage I (initial rifting period), rifting stage II (rapid subsidence period), rifting stage III (fault-depressed diversionary period). Partition of the west and east in tectonic activity was obvious. The west area showed relatively stronger tectonic activity than the east area, especially during the rifting stage II. Episodic rifting and lateral variations in tectonic activity resulted in a wide variety of structural slope break belts, which controlled both the sequence architectures and interval makeup, and strongly constrained the development of special facies zones or sand bodies that tended to form hydrocarbon accumulation. This paper classifies the genetic types of slope break belts and their relevant sequence stratigraphic patterns within the Fushan sag, and further discusses the tectonic evolution controls on sequence stratigraphic patterns, which suggests that vertical evolution paths of structural slope break belts and relevant sequence stratigraphic patterns as a response to the Paleogene tectonic evolution were strongly controlled by sag margin types and lateral variations of tectonic activity.

Identification methods of coal-bearing source rocks for Yacheng Formation in the western deepwater area of South China Sea

Owing to the fact that the coal-beds are with the characteristics of multi-beds, thin single-bed, rapid lateral changes and deep burial, coal-bearing source rocks are difficult to be identified and predicted, especially in the lower exploration deepwater area. In this paper, a new integrative process utilizing geology and geophysics is proposed for better predicting the distribution of coal-bearing source rocks. Coal-beds were identified by the logging responses of “three higher, three lower, and one expand” and carbargilite were recognized by the characteristics of “four higher and one lower”. Based on the above logical decision, coal-beds and carbargilite can be distinguished automatically by cluster analysis of logging curves in verticality. Within the constraints of well-seismic calibration, the coal-beds group also can be detected in horizontality by the integrated representation of “negative phase, higher Q, lower impedance and lower frequency” within the seismic data. However, the distribution of coal-bearing source rocks utilizing geophysical methodology may do not conform to the geological rules of coal accumulation. And then the main geological controlling factors of coal accumulation are comprehensively analyzed as follows: (1) Paleotopography and tectonic subsidence determine the planar range of terrestrial-marine transitional facies markedly; (2) The relative sea level changes affect the accommodation space and shoreline migration, and limit the vertical range of coal-beds. More specifically, the relationship between the accommodation creation rate and the peat accumulation rate is a fundamental control on coal accumulation. The thickest and most widespread coals form where those two factors reached a state of balance; (3) The supply of autochthonous clasts and the distance between deposition places and paleovegetation accumulated area are the critical factor to form abundant coal, which means that if deposition area is close to paleouplift, there would be sufficient organic matters to form abundant source rocks. The results show that the integrated methods can significantly improve prediction accuracy of coal-bearing source rocks, which is suitable for early exploration of western deepwater area of South China Sea.

Sequence stratigraphy and sedimentary facies of Lower Oligocene Yacheng Formation in deepwater area of Qiongdongnan Basin, Northern South China Sea: Implications for coal-bearing source rocks

For unveiling coal-bearing source rocks in terrestrial-marine transitional sequences, the sequence stratigraphic framework and sedimentary facies of Lower Oligocene Yacheng Formation of Qiongdongnan Basin were investigated using seismic profiles, complemented by well bores and cores. Three third-order sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center, namely SQYC3, SQYC2 and SQYC1 from bottom to top. Coal measure in Yacheng Formation of Qiongdongnan Basin were deposited within a range of facies associations from delta plain/tidal zone to neritic sea, and three types of favourable sedimentary facies associations for coal measure were established within the sequence stratigraphic framework, including braided delta plain and alluvial fan, lagoon and tidal flat, and fan delta and coastal plain facies associations. Results shown that, in the third-order sequences, coal accumulation in landward areas (such as delta plain) of the study area predominantly correlates with the early transgressive systems tract (TST) to middle highstand systems tract (HST), while in seaward areas (such as tidal flat-lagoon) it correlates with the early TST and middle HST. The most potential coal-bearing source rocks formed where the accommodation creation rate (Ra) and the peat-accumulation rate (Rp) could reach a state of balance, which varied among different sedimentary settings. Furthermore, intense tectonic subsidence and frequent alternative marine-continental changes of Yacheng Formation during the middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. The proposed sedimentary facies associations may aid in predicting distribution of coal-bearing source rocks. This study also demonstrates that controlling factors analysis using sequence stratigraphy and sedimentology may serve as an effective approach for coal-bearing characteristics in the lower exploration deepwater area of South China Sea.

Tectonic and Sedimentary Characteristics and Evolution of the Hefei Basin, China: Implications for Jurassic Hydrocarbon Source Rock Development

The Hefei Basin is an intracontinental basin located at the junction of three major tectonic units, the North China plate, the Dabie orogen and the Yangtze plate. This unique tectonic background controlled the structures and sedimentary sources for the Hefei Basin. The tectonic framework in the Jurassic is typical of foreland basins and can be further divided into wedge top, foredeep, forebulge and post-uplifting units. During the Jurassic, the main sedimentary system in the basin consisted of alluvial fan, fan delta, lacustrine delta and lake, characterized by zonation and controlled by the structural belts. Hydrocarbon source rocks in the Hefei basin were mainly developed during the early to middle Jurassic, and were influenced by depositional setting and tectonic activities constrained by the basin structural framework, depositional environment and paleoclimate. The target source rocks are only associated with the semi-deep lake zone in the lower uplift of the foredeep.