The Study Of The Karst Reservoir's Regularities Of Distribution For Ordovician In South Tahe Oilfield

The main studying karst reservoir is that in the eight and eleven block's of the Tahe oilfield. In those areas, the lithology of Ordovician are mainly carbonate and the oilpool in those regions is the karst- fractured-cavity with highly heterogeneity. The studying areas located in the bestrow of the upper Ordovician. Recent years, studies focus on the exposure of the upper Ordovician and educe many widely accepted conclusions. But to the karst reservoir of the coverage areas, many different viewpoints are still exist. Therefore, discussing the karst reservoir's developing pattern and spreading rule of the coverage of the upper Ordovician has a vital significance to the disposing of the next exploitation and development.Aiming at the fractured-cavity reservoir(mainly are karst-cavity reservoirs) in the eight and eleven block of the Tahe oilfield, All kinds of methords has been used and it contains: synthesis data concluding geology,logging and seismic, utilizing theories and techniques of sedimentary,geology of logging,reservoir geology,seismic stratigraphy,and exerting the advantage of multi-subjects study the carbonate fractured-cavity reservoir.Starting from the background of the karst action,analyzing the regions depositional background and structrue, studied the lithology and characteristic of lithology and built up structure of synthesis lithology section in the work area and lithology section frame of buried hills. All of this are good foundations to the next research. At the same time set up the identify symbol of karst action, researched the karst action and discussed the spreading rule of the karst reservoir. According to the synthesis analysis,the mainly conclusions are as follows:1,Based on the understood of the Ordovician's characteristic of sedimentary and lithology, combined with the former conclusions, the buried hills lithology section of south Tahe oilfield has been built by the date of logging. Simultaneity, according to the differentiate ability of the karst, the buried hills section can be divided into four layers in vertical. The plane distribution extension of different buried hills sections has been fixed on and carved up four zones. This work made a good foundation for the correctly realize and analysis of the karst action and karst reservoir in the south Tahe oilfield.2,On the basis of the study for the diagenesis sequence of the karst phenomena, characteristic of the carbon, oxygen and Strontium isotope in the result of the karst and the macroscopical distribution of the karst cavity layers, the timing of the karst action has been assured. It is pointed that the karst phenomena and the karst fractured and cavity of coverange of the upper Ordovician in the south Tahe oilfield are still the result of the karst of Hercynian. The karst and erode action of the parallel unconformable surface of the middle Caledonian are little operate to the reservoirs in those area.3,It is concluded that dominated by the unconformable surface of Hercynian the south Tahe oilfield still developed three karst cycles and three sorts of cavities based on the identify of karst cycles on the single well section and the comparison of the layers of cavities in the background of early Hercynian. There are completely three karst cycles and three sorts of cavities in the north of the eight block of Tahe oilfield, so as to the further north and east. The south of the eight block conserved karst cycleⅡandⅢ, and two sorts of cavities. The eleven block of Tahe oilfield only conserved karst cycleⅢand one sort of cavity. The karst action developing has a strong heterogeneity. The combination of the cycles on the single well section contains:Ⅰ+Ⅱ,Ⅰ+Ⅲ,Ⅱ+Ⅲ,Ⅰ,Ⅱ,Ⅲand the highest probability on the single well section is single karst cycle and cycleⅢ. As to the complanate distribution of the karst zone, from karst cycleⅠ→karst cycleⅡ→karst cycleⅢ, the zone of the karst slope expand to the east ,south and west continuely. The distribution of the karst zone has a relatively strong inheriting in the stage of the karst cycleⅠandⅡ. But the distribution of the karst zone in the stage of karst cycle III is different from the former one.4,The karst in the early stage of Hercynian is still follows the " karst model of normally water table under the atmosphere water" and the "karst model of atmosphere water deep undercurrent dominanted by fault". In this paper, the karst model have been perfected, especially for the later one. At the same time, nailled down the range of time and space of the two karst model to a further step. It is pointed out that the main area and the eight block developed three karst cycles and the karst and obeys the " karst model of normally water table under the atmosphere water". However, the eleven block of Tahe oilfield mainly developed karst cyclyⅢ, and obeys the "karst model of atmosphere water deep undercurrent dominanted by fault".5,Understood the karst stage of Caledonian, it is indicated that the parallelism unconformable surface of the I action in the middle stage of the Caledonian dominated the karst of atmosphere water which may developed in wide areas of southern Tahe oilfield in middle Ordovician. But the atmosphere water dominated by the II action in the middle stage of the Caledonian parallelism unconformable surface which formed in the upper Ordovician formation of Lianglitage only developed in parts areas and the karst effection of the III action in the middle stage of the Caledonian in late Ordovician has little effect.6,The karst of Caledonian mainly composed the karst-cavity reservoir. Because of this, under the parallelism unconformable of the second act in middle stage of Caledonian at the same time at the top of Yijianfang formation the range of 10-40meters, there may developed karst-cavity reservoir of wackstone and in the plane widely developed in the carbonate platform of Yijianfang formation. The kiesel crust of weathering developed in the range of 10-30 metres on the top of Yijianfang formation. Under the parallelism unconformable of the second act in middle stage of Caledonian at the same time at the top of Lianglitage formation the range of 0-30 meters, there may developed karst-cavity reservoir and in the plane, it is developed in the top of Lianglitage formation in the facies of carbonate platflom boundary of the high standed tract.7,By the analysis of the reservoir of the karst- fractured -cavity in the early stage of Hercynian, it is made clear that the karst- fractured -cavity reservoir in the early stage of Hercynian mainly developed in formation of Yijianfang , middle Ordovician and the formation of Yingshan, lower Ordovician. The occurance has three types: The first one is developed in the unconformable surface of early stage of Hercynian and its neighborhood. The second one is in the top, bottom cavity which developed in the early stage of Hercynian and interlayers. And the third one developed inner of Ordovician singly. The first two is the mianly types. The structure of buried hills lithology section effect the developing distribution of the karst-crust-cavity reservoir in the early stage of Hercynian and this kind of reservoir has a strong heterogeneity. The developing condition of the karst-crust-cavity reservoir in the eight block of Tahe oilfield can be divided into three zones with different characteristic: the first is in north of the wells line of T808-TK838-TK839-TK232 north, the second one betweens the wells line of 808-TK838-TK839-TK232 and TK847-TK828-TK844-TK843, and the third one is in the west wells line of TK847-TK828-TK844-TK843. 8,Analysing many factors affect to the developing and distributing of the karst reservoir systematically and those factors contains of lithology, faulting vs split system, palaeogeomorphology and uncomfortable surface. It is concluded that the lithology affect the layer of karst developing. Buried hill structure of lithology section impacts the intension of karst and the developing density of cavity, the behavior manner of karst action and the characteristic of vertical distribution. Frctures affect the forming and developing of reservoir in three aspects. Firstly, the frcture zones are the good areas for the karst. At the same time the tend towards of the developing cavity is the same as the frcture zones. Secondly, the former frctures offered the cannel for the karst of Hercynian and dominated the depth of the karst. The main frctures dominated cavities in the deep area and small frctures dominated middle-deep cavity. Karst palaeogeomorphology has advantage and disadvantage affections for the developing of karst reservoirs. The particularity of the karst palaeogeomorphology and its microtopography played a dominated way for the idiographic developing of the karst cavities. Unconformable surface is the surface of structure weakness. It is offered a good condition for the vertical developing of the karst and the gapping time of the unconformable surface affect the intensity of karst and the degree of reservoir 's developing.