Character of Terrestrial Sequence Stratigraphy and Depositional System in Incised Valley, Outcrop Area of Karamay Oilfield, Junggar Basin, China

In the Karamay oilfield located on the northwestern margin of Junggar basin, Xinjiang, China, a large area of the Karamay Formation is exposed at outcrop in the northeast of the oilfield, a consequence of thrusting. The Middle Triassic Karamay Formation in the outcrop area is a type of terrestrial third-order sequence, bounded by two easily recognizable sequence boundaries: a regional surface of angular unconformity (SB1) at the base and a regional unconformity (SB2) at the top. Within the Karamay Formation, two lacustrine expansion events can be recognized and be used to identify both the initial and the maximum lacustrine flooding surfaces. The two lacustrine flooding surfaces serve as references for the classification of this third-order sequence-Karamay Formation into the following three sedimentary successions: a lower lowstand systems tract (LST), a middle lacustrine-expanding systems tract (EST), and an upper highstand systems tract (HST). Different systems tracts are composed of different depositional system assemblages. In this paper, each depositional system is described in detail. The lowstand systems tract in the study area is characterized by incised valleys. At the base and on the margin of the incised valleys occur alluvial fan depositional systems, and in the upper and distal parts of the alluvial fan, low-sinuosity river depositional systems. The lacustrine-expanding systems tract consists of a lacustrine depositional system and a lacustrine delta depositional system, overlying the lower incised valley fills. The highstand systems tract is filled by a widespread lacustrine braided delta depositional system. The analysis of sequence stratigraphy in this paper serves the description of the spatial distribution of the reservoir. The depositional system analysis serves the description of the reservoir types. Field investigations of oil sandstone and oil seepage show that the Karamay Formation is composed of several types of reservoirs. However, two types of high quality reservoir occur both in the upper interval of the lowstand systems tract and in the lacustrine-expanding systems tract: gravelly low-sinuosity channel in the distal fans and sandy-gravelly distributary channel in the lacustrine delta plain.

Geochronology and Petrogenesis of the Pillow Basalts in Western Karamay from West Junggar,NW China

Recently, we carried out the National Nature Science Foundation of China （No. 41303027） and focused on pillow basalts in western Karamay from West Junggar. The detailed geological survey at a scale of 1：500 distinguished an assemblage of accretionary complex in western Karamay. The accretionary complex consists of pillow basalt, mudstone, turbidite and abyssal radiolarian chert interlayered with tuff （Fig. 1a）. The great concern in the study area is the large-scale pillow lava.

Control of slope-pattern on the deposition of fan-delta systems:a case study of the Upper Karamay Formation,Junggar Basin

The Mesozoic fan deltas in the north-west margin of the Junggar Basin,as important petroleum reservoirs,exhibited complex facies change and internal structures with strong heterogeneity which were controlled by the transformation of slope-patterns,bringing great challenges to the study of sedimentary characteristics.The Upper Karamay Formation at north-west margin of the Junggar Basin was the objective in this paper which attempts to clarify the mechanism of sedimentary response and sand-body distribution of fan delta systems under the control of slope-pattern change.Based on a data set of cores,well logs and seismic,two types of slope-pattern were identified in the study area,which include steep-to-gentle in the south and gentle-to-steep in the north.The control of difference slope-patterns on the sand-body distribution was clarified based on the analysis of the sedimentary dynamics,facies characteristics,and depositional evolution of the fan deltas.The study shows that the transport mechanism of sediments on the steep-slope was dominated by debris flows,developing coarse-grained,thick-bedded lobes with poor structural maturity of clasts.On the gentle-slope,the deposition was dominated by hyperconcentrated-traction currents,forming relatively fine-grained,thin-bedded lobes with increased sandy matrix.The sand-bodies show frequent bar-channel transformation and channel down-cutting under the steep slope setting,which exhibit migration of isolated river channels on the gentle slopes.Under the steep-to-gentle pattern,the coase-grained sediments were mainly accumulated at slope toe,generally developed equiaxial lobes.However,the coarse-grained clasts were preserved both at proximal and distal lobes on the gentle-to-steep slopes,showing obvious lateral extension of the fan delta.The slope patterns controlled sedimentary respond rates of the fan deltas during lake level change.By comparing the modern cases of fan systems worldwide,the control of slope patterns on deposition of coarse-grained fans was clarified,providing insight into hydrocarbon exploration on basin margins.

Stratigraphic Framework and Microfacies of the Triassic Lower Karamay Formation in Districts Ⅰ,Ⅲ, NW Junggar, China

The Triassic Lower Karamay Formation（T2k1） is one of the main oil-bearing stratigraphic units in the northwestern margin of Junggar Basin（NW Junggar）, China. Based on an integrated investigation of outcrops, well logs and seismic data of NW Junggar, the Lower Karamay Formation is subdivided into 2 sets, 6 beds and 13 layers. Also, it is considered that the alluvial fan, the braided river, the meandering river and the shore-shallow lacustrine were developed during the early period of Middle Triassic in Karamay districts Ⅰ, Ⅲ. The Lower Karamay Formation deposits the following 9 sedimentary subfacies： the lower fan, the middle fan, the upper fan, the braided channel, the overflow bank, the river flood of braided river, the meandering channel, the river flood of meandering river, the shore-shallow lacustrine. Seventeen microfacies such as the braided stream channel, the alluvial sand floodplain, the alluvial glutenite floodplain, the water channel, the channel bar etc. have been identified in subfacies as well. The thickness of strata is firstly up-thinning and then thickening upward to the top, it is a process from large-scale lacustrine transgression to partly lacustrine regression. Furthermore, the microfacies planar distributing law proves that the remaining oil may enrich along the margin of microfacies because of the planar microfacies changes. Therefore, this research is beneficial for searching remaining oil in NW Junggar and providing information to the project ＂The Secondary Exploration＂ of PetroChina.

GEOCHEMISTRY OF PETROLEUM MIGRATION DURING THE FORMATION OF KARAMAY OIL FIELD

The phenomena of geochemical migration during the formation of the Karamay Oil Field have been revealed by correlating the compositions of non-hydrocarbon, asphaltene, steranes and terpanes from between oils and rocks. The study on abnormal compaction observed in a deep well, which is located between the source area and the oil field, and the corresponding depletion of hydrocarbons further clarifies the conditions of the migration of oil and gas. Results show that the necessary condition for the formation of this giant oil field is a long distance of lateral migration of oil and gas followed by a redistribution along Ka.-Ur. large fault before accumulation in a large uplift.