3D Geomodelling Of Karst Conduit In Ordovician Of LunGu Oilfield

Marine carbonate occupies a very important position in the world oil and gas production. According to statistics, there are more than 300 distributed in marine carbonate of the 945 founded large oil fields,47% of total reserves; global carbonate oil and gas production accounted for about 60% of world oil and gas production. But at the same time, it’s the most difficult reservoir to be forecast.In recent years, with the deepening understanding of geological and exploration technology, marine carbonate in China has entered the important oil and gas exploration and discovery period. Ordovician carbonate oil and gas exploration have achieved a major breakthrough in north and middle area of Tarim basin, such as Tahe oil field, Lungu oil field, Tazhong area, Ordovician Yingshan group also received a major breakthrough, all of these make the Tarim basin carbonate has become one of the most important oil and gas exploration areas of China.Tarim Oilfield Lungu is currently one of the large -scale carbonate reservoirs oil fields in China. The key problem of restricting oil exploration and development of is that the carbonate chimney cave system development and distribution of prediction.Reservoir exploration and development key issues are the three fractured karst system with the identification and extent of distribution. In recent years, carbonate reservoir exploration technology have attracted universal attention and is also seen in the exploration of a certain effect, breakthroughs in key technologies have not yet been made.Therefore,3d geomodelling of karst conduit in ordovician formation LunGu oilfield, especially for the karst seam-cave reservoir system and the research methods are needed to be discussed.For the development of Chinese Marine carbonate oil and gas geological theory, it will effectively promote the carbonate oil and gas exploration and development to determine the tarim carbonate oil and gas exploration direction.In this paper, we select the ordovician of LunGu oilfield in tarim basin as the key anatomical object, using material of seismic, geology, well logging, mud logging and physical properties, carbonate reservoir geology theory as the instruction, combined with the theory and technology of karst hydrology, on condition of the crack model and inception horizons,looking for the 3d geomodelling method of integrated multidisciplinary complex karst conduit system.This method will provide important theoretical guidance and technical support for exploration and evaluation of seam-hole reservoir in the tarim.Closely track modeling international frontiers and hot topics, the study of karst conduit system modeling will not only promote the seam-hole carbonate reservoir research to quantitative and intensification in the direction of development, but also rich theory and application of reservoir modeling technology.All of this has significant theoretical and practical significance.The main known as follows:1. The regional stratigraphic occurrence affect the growth of karst caveIn the work area to track the conduit on the seismic section,we can found that part of the conduit is bedding foliation, that is obviously controlled by strata occurrence.This conduit is the same as Ford’s four state model.From the perspective of groundwater dynamics, groundwater movement in the phreatic zone controlled by regional drainage datum and strata, so the strata dip azimuth information must be needed as a constraint condition for automatic tracking algorithm to ensure that the developmental direction is a correspondence between karst cave and construction situation.2. High linear density stripe instructs the developmental direction of karst conduitLinear density is a kind of parameters that reflect the linear structure intensity such as in the fault and fracture.At first, the parameters are used to analyze mineral and energy occurrence and engineering geology and seismic geological conditions, today we use it to predict connecting the cave paths between the water in the cave and karst springs.The method based on karst conduit will give preference to rock crack high degree of development path, thus we can infer that the high linear density can indicate the existence of karst conduit.Unlike nowadays’s karst, in this paper, the linear density method is introduced into predicting karst conduit of ancient karst system.The method involves some parameters, such as determining water entry and drain position, strata dip azimuth information and initial linear density DL, are unable to use technical means to obtain, so in the paper we modify parameters accordingly on the basis of the existing data.(1) using the actual application in drilling of well point or seismic profile of cave, there are obvious beaded reflection characteristics of points, as the default conduit "starting point" and "end" or constraints.(2) using recovery under the background of karst palaeogeomorphology formation level inclination and azimuth attributes to reflect the characteristics of the ancient karst geological structure.Due to the ordovician top surface (Tg52) by weathering erosion and atmospheric fresh water leaching and other physical chemistry effects,it cannot reflect the structure of the karst formation during the period of form, so we choose the interface (Tg53-1) to extract the information of stratigraphic dip azimuth.(3) taking the ant tracking method to obtain the initial linear density DL.For ant tracking data body along the layer (Tg53-1) instantaneous amplitude attribute extraction, we build a grid that is 20 mx20m, then the grid attributes as the initial linear density DL.High linear density value can only be on behalf of the karst conduit more likely development along the strip, so linear density method to predict karst conduit need other parameters.In view of the present seismic attribute to predict conduit with subjectivity and uncertainty, this paper come up with a new method that based on ants tracking properties,the stratigraphic dip azimuth as constraint conditions.On the basis of the seismic amplitude attribute predicting conduit, specifically for a certain period of conduit that attributes describe,we select time depth section to calculate linear density, then we can amend and perfect the amplitude attribute to predict conduit, to make the result of the prediction more accurate.3.Establish a principle of inception horizon in LunGu oil fields Zhongxiepo area and divided into three inception horizonsLowe think that in the phreatic zone, karst conduit develop belong the limited number of sedimentary formation, the particular sedimentary strata is called inception horizon.The division of the inception horizon needs to follow the following principles:(1) selection standard horizonStandard horizon is relatively stable formation after karst development, so that calculate the distance between the cave and standard horizon can accurately reflect the characteristics of cave development in karstification period, then we can effectively distinguish the stage of karstification. The cave of drilling in Zhongxiepo area is located in the Yingshan formation,through analysis the seismic horizon in Yingshan formation.we select Tg53-2 for the standard horizon.The horizon lithology is pure limestone, relative to the Tg5 and Tg53-1 is more stable, relatively stable formation after karstification,that can identify on the seismic trace.(2) calculated the vertical distance between cave and standard horizonWe adopt the method of drilling, logging and seismic recognition identify cave to obtain cave that the scales larger than 2 m statistics.The bottom of the cave than ceiling can better represent the inception horizon accurate location.Computing Tg53-2 (standard horizon) stratigraphic dip, we can obtain the vertical distance between the bottom of the cave and standard horizon.We can drawn cave scale map by statistical results related to the vertical distance.The three peak in the figure compared with uniform distribution in the strata has a higher density of the cave, the cave peak representative karst development concentrated horizon, for the three inception horizon of karst development in the study area.The inception horizon located in Tg53-2 bottom up 65 ms, next to 85 ms, the longest is 105 ms,then we can establish the inception horizon by choosing Tg53-2 3 hours window.4. The inception horizon and caves have good corresponding relationshipThrough analyzing karst hydrology in Zhongxiepo area, according to the ordovician karst landform erosion terrace, river main crack point and the upper carboniferous strata, combining with the characteristics of the karst geomorphology, we divided five karst platforms, and research work area are mainly distributed inplatform 2, platform 3 and platform 4.The three karst platforms relative cave layer is consistent with the three inception horizon, namely the three inception horizon respectively corresponding to the first cave, the second layer and the third cave.All of these show that the inception horizon and caves have good corresponding relationship.5. setting up crack stochastic model of research work areaThis study selected Tg53-2 horizon to reflect the early hercynian paleostructural shape in LunGu area, namely the Tg52 surface as the top surface of structure model, and the Tg53-2 as the bottom surface.Using ant tracking identification of fracture, eight iteration ant tracking results at the same time, so as to reduce the discontinuous ants body, then it keeps large fault information, which reflects the main fracture and fracture trend.With variance, maximum negative curvature and the distance from the fracture by the use of neural network clustering analysis,we can calculate properties and variation function by kriging interpolation method to the fracture density in the study area.Inputting the date into crack modeling software, calculating the multiple discrete fracture network model, we can pick out model that is the most conforms to the actual situation.6. Programming to achieve the shortest path between the specify two points by extracting minimum energyTransforming the line of fracture network for gray image, making its show linear characteristics of image background, we can obtain the minimum energy path between two points using the shortest path method with the linear moving fast.This method only needs the characteristics of seed points as prior knowledge, then the extracts continuous linear feature objects of crack. This method belongs to the semi-automatic extraction method.At the same time, in order to improve the extraction efficiency, we must make effective saddle points to achieve the purpose of human intervention results.7. The comprehensive seismic amplitude attribute, linear density results and random discrete fracture network model to predict karst caveUsing seismic amplitude attribute, linear density results and random discrete fracture network model to predict karst cave has a hierarchical concept: at first, by using seismic amplitude attribute can distinguish the conduit;Then for conduit that couldn’t identify under the seismic resolution, we can determine connecting path between the high value by contrasting the zonal characteristics of linear density;Due to the linear density represents the development trend of karst conduit, and the fracture represents a smaller scale, with random manual intervention results, according to the random discrete fracture network model through fast marching algorithm simulation for tracking can be more detailed describe properties of discontinuous conduit route on the map.8. Use combing seismic attribute to calculate the 3D modelling of karst conduit in work areaCombine RMS, instantaneous frequency and instantaneous phase attributes to a mixed-volume attribute, constraining by the path of karst conduit, built a 3D geologic model of karst conduit. From the model, the cave which has a bigger size has a better consistency than a smaller one.The paper combines the advantage of mature theory model in karst hydrology and 3D visualization interaction in geophysical method. Karst conduit genesis follow the minimum energy, so it is possible to use many available data to constrain the model of karst conduit to make the result more accurate. There are two main innovations in this paper.1. Introduce lineament density which reflects the density of lineament structure to the prediction of karst conduit and propose lineament density method which based on fracture information calculating by ant-tracking and constrained by anisotropy of fracture and dipping-azimuth of bedding plane. Classic prediction of karst conduit method using seismic amplititude attributes have a incontinuity feature when a conduit have a size below 10 meters, so the result have a subjectivity and uncertainty. Based on the prediction of cave conduit by seismic amplitude attribute, using lineament density to correct the result predicting by seismic attribute, make the result more accurate and effective.2. Some specific horizons play a very important role in karst conduit genesis. These horizons are called inception horizons. According to cave size and distance to standard horizon, there are three inception horizons in work area. Basing on fracture modelling and constrained by karst hydrology and inception horizons, using fast marching method to calculate the minimum energy. Combing seismic amplitude attributes and lineament density method, further refine the path of karst conduit.