Deep Permian Tectonic Characteristics、Basin Prototype And Source Rock Evaluation In The Northwest Tuha Basin

The petroleum exploration of the Tuha basin in recent years indicate that there is a good prospect in the deep Permian. For example, the well Tuocan 1 in Lukeqin oilfield has confirmed that the source rocks from Permian own a good exploration potential. Among the totally proved reserves at 462 million tons, there is nearly 100 million tons deriving from Permian source rocks. The outcrop from Taerlang and Aiweier channel in the northwest of Tuha basin exposed well Permian source rocks. However, the level of exploration for the deep formation is still very low. Many basic Problems of the petroleum geology still need to be resolved, such as stratigraphic distribution, sedimentary features, prototype and evolution of the basin, distribution of the source rocks and so on. All this problems are the important issue to be solved for the deep petroleum exploration and prediction in the Tuha basin.This thesis aims at analyzing the structural characteristics, basin prototype and hydrocarbon potential of the source rocks of the deep Permian in Daheyan block, northwest Tuha basin. Under the tectonic background of Tuha basin and based on the previous research on gravity, magnetic, electrical and seismic profiles, outcrop, drilling, logging and so on and making fully use of the result of electrical and seismic profiles, filed geological survey. During this research process, the integrated geological interpretation techniques combing geology and geophysics, balanced section analysis method for tectonic evolution, basin analysis on the prototyping and evolutionary and oil-source correlation techniques were combined together. With the help of these technology, the structural interpretation and stratigraphic distribution characteristics of deep Permian were completed, the structure and tectonic framework of the deep Permian geological were determined, the tectonic evolution process was also analyzed. What is more, according to the stratigraphic distribution, exhumation thickness and feature of sedimentary facies, this thesis discussed the prototype and evolution of Tuha basin, even the hydrocarbon potential, development environment and background of the deep Permian. This research has important guiding value for the next deep petroleum exploration in the Tuha basin.Based on the analysis on the regional tectonic background, this thesis make a comprehensive geological interpretation on 5 MT profiles. Combining with the interpretation outcome on gravity, electric method, and seismic profiles, the structural boundary of the northwest margin of Tuha basin was determined and found the faults between the basin and the mountain are well developed. The faults are main structure in this research area, meanwhile the dividing line of the basin and the mountain. The boundary of basin or the contact relation between the basin and the mountain are comprised a group of faults and overlap contact line. The boundary is inadequately fixed on any fault, but very complex and transforms among faults such as F2, F3 and F4. According to the stratigraphic sedimentary and tectonic features, this thesis make a new division on the tectonic units.This thesis make research on the seismic interpretation of the Permian and get the new structural figures. Selecting the Permian as the objective layer, combining the structure feature on the structure and strata, this thesis put forward the structural model. The horizon calibration was based on data such as synthetic seismograms, VSP, outcrop and overlay offset. Guided by the structural model, based on the seismic interpretation and instantaneous profile, the fault was identified. Through the work on the gravity and seismic inversion, the interoretation model was verified. The velocity analysis carried out by the model chromatography, after which the time-depth conversion and structural figures were completed. The structure diagram of the middle Permian gain during this research lay the foundation for the work of the distribution of the residual formations, exhumation thickness and recovery of the original stratigraphic thickness.Through the analysis on the depth structure map, the thickness distribution of the residual strata of Permian of the study area was gained. The residual thickness of the lower Permian in Tuokexun depression is up to 600m. Because the development of Bogeda rift, the sedimentary thickness reach its maximum of more than 1200m.In general, the sedimentary thickness is thin in the south of Daheyan city, which average is of 400m, however, in the north of Daheyan city, to the direction of Taoyuanshu, the strata thicken quickly. The residual thickness of the middle Permian only clumped distribute in the Keyayi depression and Tuokexun depression, and form a sedimentary center respectively in these two depression. The residual thickness is about to 400-1400m in Tuokexun Sag, the maximum thickness can reach to 1500m, and the residual thickness is between 600-1000m in Tuokexun Sag.Based on the research on the theory and method of the exhumation thickness, through trend extrapolation on seismic stratigraphic, this research restored the exhumation thickness of the middle and upper Permian and reveal the erosion characteristics of the study area. The research outcome show that the exhumation mainly occurs in Taibei depression, and the exhumation thickness increase gradually from north to south, after then decrease to the state with neally no deposition. The exhumation thickness reach the top at 800m at grape valley and Well Shanke 1. The exhumation on middle Permian mainly distributed at Taibei depression, Kalatu uplift, Keyayi depression and Tuokexun depression. There are two exhumation thickenss pinch in the north of Taibei depression, with an increasing trend from north to south and reach the peak in the slope, then decrease to the south. The exhumation thickness reach the peak value at more than 1200m in the Kalatu uplift.According to the characteristics of residual stratigraphic thickness and exhumation thickness, this thesis recover the original thickness of Permian in the study area. The results show the residual thickness is in accord with the original thickness of the lower Permian series. The original thickness of the lower Permian increased gradually from south to north in the northwest edge of Tuha Basin, form 600 to 1600m. And there is no lower Permian sedimentary in the north of Buerjia Rise of our reach zone. The middle Permian sedimentary is mainly in Taibei depression, Keyayi depression, Tuokexun depression. The thickness is reduced gradually from north to south in the northwest edge of the basin, which from 1400 to 200m. In the Keyayi depression, the thickness decrease from east to the west, which change from 1600m to 800m. The thickness is increased from east to west in the Tuokexun depression southeast of the study area, from about 200 to 1800m. While there is no deposition in the Buerjia Rise. The upper Permian mainly deposit in Taibei Sag and its vicinity, the thickness of deposit reaches a maximum value, more than 1400m. In Daheyan area, the thickness of upper Permian reduce gradually from northeast to southwest, mainly distributed in Buerjia uplift and Kalatu uplift.This thesis studied the Permian tectonic evolution by the analysis of fault activity and restoration of equilibrium profile. It is considered that the Permian mainly with the structural feature controlled by basement rift, controlled sag-uplift palaeogeographic pattern and the evolution can be divided into three stages, which are stretching stage after the collision during early Permian, reversion stage in middle Permian and developmental stage of analogous foreland basin in Late Permian. Tectonic evolution gets the following rules in three aspects. The tectonic episodic evolution is mainly by uplifting. The tectonic episodic evolution has controlled the limitations of stratigraphy sedimentation. The multiple uplifting resulted in the complex tectonic framework. According to the tectonic movement period time, plane of unconformity, deformation behavior and stratum combination, the Daheyan region of Turpan-Hami basin can be divided into 4 first-order sequences which is N2-Q、J-N1、P2-T and C-P1, and other 7 second-order sequences and 12 third-order sequences.By analyzing the Permian stratigraphic rock combinations and sediment characteristics, this thesis analyzed the depositional environment and system in the study area. The result show that during Lower Permian, it is mainly developed with alluvial fan, fluvial and lacustrine facies, littoral facies, bathyal-abyssal facies, this shows the area should be the deep rift basin environment. The middle Permian of Daheyan formation is overall a syntectonic period rapidly deposited near source fan and fluvial facies, which are mainly conglomerate and conglomeratic sandstones, and lack the muddy deposition. During the Tarlong sedimentary period, the Tuha basin and Junggar basin has similar sedimentary environment, the water may combined with each other, and form a unified sedimentary system. The tendency of the sedimentary system form the edge to the center is:alluvial fan, braided river delta, shore and shallow lake, half deep lake-deep lake facies.Based on the research of strata distribution, exhumation thickness and the sedimentary facies, the thesis discussed the prototyoe and evolution of the basin. The outcome show a territorial crustal spreading is taken place in the west of China during Early Permian. Because of this crustal spreading effects, a series of Early Permian rift basins developed in Tuha Basin and adjacent regions, as well as several times volcanic eruption. In the north-western of Tuha Basin of the research area generally developed pelite-lava sedimentary assemblage of inland rift faults basin. At the basis of rift deep basin in Early Permian, the Tuha Basin evolved into rift depressed basin gradually in Middle Permian, finished the evolution from Early Permian deep basin to Middle Permian residual epicontinental sea basin, and transformed into lacustrine facies in the middle-late Middle Permian. Tuha Basin got into the period of the development of terrestrial lacustrine and the basin transformed from rift basin into Sag basin which started since Late Permian. The mainly plate movements of structual evolution in Paleozoic was attributed to the close of ancient Asia Ocean.The result of geo-chemistry analysis of the outcrops of the eight profiles and the characteristics of the source rocks of Middle Permian Tarlong formation from drilling wells says: the TOC of the eight profiles outcrops average 3.72%, the triangle figure showing the content of C27-C28-C29 sterane indicate that the main type of the organic type are Ⅱ1 and Ⅰ, which show the mature feature. The source rocks here are good and has a high hydrocarbon potential. While the composite analysis of drilling source rocks in adjacent regions shows:it’s TOC average 1.86%, chloroform asphalt "A" average 0.0158%, S1+S2 average 2.2mg/g. The kerogen is mainly type Ⅲ, type Ⅱ2 takes 8.70% and there’s little type Ⅰ exist. Which could be evaluated as middle-poor source rock. The structure-sedimentation pattern and the influence of the sedimentary environment are the reasons caused the differences. The main part of the outcrops of the northwestern Tuha Basin locates on half deep lake-deep lake facies sedimentary regions, while adjacent regions mostly located on shoreline-shallow lake or braid lake delta sedimentary regions. This difference on sedimentary environment lead to the difference of stratum lithology and further more affects the quality of source rocks. Overall, the research region of northwestern Tuha Basin is on a good source rock regions. The effective hydrocarbon source rocks of Middle Permian Tarlong formation are mainly distributed over Tuokexun and Keyayi Sag, which have a effective area of 670km2,200m-400m thick, having a Ro range from 0.62% to 1.15%, evaluated as mature-good source rock, with an high hydrocarbon potential.This thesis also make research on the source rock of Taerlang formation in Permian and the relationship between the structural features and prototype of the basin in the study area. The result indicate the area of northwest of Tuha Basin belongs to Bogurda- Qijiaojing rift valley, which was the largest one in the Permian. Terrigenous materials formed in the intracontinental rift, containing rich organic matter, became the most important source rocks in this area. In the middle period of the Middle Permian, the Tuha basin gradually change from deep water rift basin of the early Permian into depression basin. The mainly characteristics are weakness of the fault, stable settlement of the basin and expanding of the lake. Moreover, the sedimentation rate of the basin in this period is still at a high level, which is suitable for the development of the lacustrine source rock. The source rocks formed in this period are mainly green-gray to ash black shale and mudstone, mixing sandstone and thin layer carbonate rocks in the lithology. The rocks were rich of freshwater animal fossils and were weak in wave action, reflecting the sedimentary environment was inland lake basin at the time, where the deep lake subfacies were relatively developed in north of Tuokexun depression. Taerlang Formation is the first important lake transgression sediment in Tuha Basin and the primary deposit scope was much bigger than nowadays, which was the favorable time of high-quality source rocks in the basin. Semi-deep-deep lacustrine facies were the sedimentary center, where the main sediment were black, dark oil-shale, shale and mudstone, with prosperous aquatic life and good preservation condition.