Detailed Research On The Reservoir Geology Of Dongfenggang Oilfield

With the continuous development of oil exploration and exploitation, subtle reservoirs, especially in basins with high degrees of exploration, have gradually become the significant prospecting targets instead of structural reservoirs that are easy to be identified. Turbidite subtle reservoir, whose reserves play an important role in subtle reservoir reserves, is one of the significant subtle reservoir types. Take a typical turbidite reservoir in the major blocks of Dongfenggang Oilfield as an example, this paper focuses on the detailed research on reservoir geology and finds a number of fans as the next prospecting targets.In accordance with its requirements, reservoir fine geologic research, must be carried out with"great care", in addition, the geological model established by fine geologic research must be"predictable."The requirements for predictability mainly include the following two aspects: one is the prediction of sedimentation and reservoir, and the other is the forecast of hosting gas.The prediction of sedimentation and reservoir lies in building a suitable sedimentary facies model of the studied area and analyzing its controlling factors'influences on the formation and evolution of sedimentary facies, while on the basis of the above work, the forecast of oil and gas bearing in the reservoir is to analyze the distribution regularity of oil and gas through reservoir heterogeneity research, providing guidance for oil exploration and exploitation.The detailed stratigraphic division and comparison as well as the establishment of high-resolution isochronous stratigraphic framework are one of the foundations and key factors in oilfield exploration and exploitation. High-resolution sequence stratigraphy combined with sedimentary cycle analysis is used to perform fine correlation and division of Sha-3 upper sub-member and Sha-2 member.On the whole, Sha-3 upper sub-member and Sha-2 member constitute a three basal level cycle. The boundary of Sha-3 member and Sha-2 member are equal to two four basal level cycle boundaries inside the three basal level cycle. There are two four basal level cycles in Sha-3 upper sub-member and one in Sha-2 lower member, all of the three cycles are ascending ones, meanwhile in Sha-2 upper member, the only cycle is a descending one. In total, 12 five basal level cycles are identified, among which 4 are in Sha-3 upper sub-member, 4 are in Sha-2 lower member and 4 are in Sha-2 upper member. The five basal level cycles in both Sha-3 upper sub-member and Sha-2 lower member are mainly characterized by ascending cycles as well as compounding cycles formed by ascending and descending ones, whereas in Sha-2 upper member, they are typical descending cycles.Following the correlation, a detailed research on the characteristics of distribution and evolution of sand bodies and sedimentary facies both in vertical and planar direction has shown that, a turbidite fan with a small scale and simple inner structure is developed on the gentle slope of fault trough lake basin in this area. It scatters randomly with a shape of thin layer and in vertical direction, multi-stage fan bodies and mud layers appear mutually.A"superimposed fan"model of turbidite fan deposition in the studied area is put forward in this paper through lots of practice. Superimposed fan refers to a distribution pattern of fan bodies, through which different-phase fan bodies superimpose on each other in vertical direction, and fan bodies take turns to move forward or backward, while fan bodies move far away from the center on the horizontal level. Superimposed fan model is featured by"fan bodies superimpose vertically, overlaying on each other; fan bodies move laterally, standing side by side"with the following three important characteristics: firstly, multi-stage fan bodies superimpose on each other in vertical direction, and different fan bodies may move away in the lateral direction; secondly, individual fan body in superimposed fan bodies is very small with simple internal structure; thirdly, superimposed fan model differs distinctively from Walker's"suprafan"model. Superimposed fan model is not a single-phase model but a complex one.The reservoir prediction direction of superimposed fan model is obviously different from that of Walker's"suprafan"."Vertical superimposition"is in the forecast of multi-stage fan bodies overlaying in the vertical direction, and"lateral movement"indicates that new fan bodies may appear on the two sides of individual fan bodies on the plane. Adhering to"superimposed fan"model, multi-phase fan bodies superimposition may be found on the upper or lower parts of the fan bodies which have been discovered, and new fan bodies may also be found on the two sides of the fan bodies which have been discovered.On the basis of analyzing the factors intriguing the formation and evolution of sedimentary facies, such as tectonic movement, lake level changes, sediment supply, it is concluded that"source -slope– depression"collocation and"lake level changes"both dominate the formation and evolution of turbidite fan. Concerning the study of turbidite fan,we first need to center on the"source", then trace the transportation path along the"slope"and look for the fan bodies in the"depression". Various"source - slope– depression"arrangements may lead to fan bodies with different characteristics. In particular, the paleogeomorphology of deposition zone caused by the deposition of turbidite fan directly controls and influences the scale and distribution of turbidite fan. The lake level change in the vertical direction controls the developmental phases and evolution features of fan bodies.Dominating by the"superimposed fan"model and reservoir heterogeneity, the oil and water distribution in studied area in the vertical direction is characterized by thin oil layer with an average thickness of 0.65-2.84m, multi oil layers and uneven distribution mainly focusing on the middle part of 1, 2 and 3 sand bed sets in the upper Sha 3, meanwhile, on the plane, single oil sand body has a small area, and scatters randomly, there are 169 sand bodies in total, among which, only 8 are controlled by more than seven wells and 119 by less than two wells. While, multi-stage oil sand bodies grow congruently in a large scale.Through researching on fan distribution regularity, reservoir heterogeneity and reservoir comprehensive evaluation, on one hand, fans overlaying on each other become the prospecting targets by analyzing the control of lake level changes on fan superimposition referring to its vertical characteristics; meanwhile,on the other hand,"lateral movement"and"source-slope-depression arrangements"provide guidance for finding fans developed in the paleogeomorphic low part on the two sides of individual fan bodies. Finally, favorable targets are confirmed by predicting favorable fans according to the results of reservoir comprehensive evaluation.Referring to the results of detailed research on reservoir geology, continue eastwardly drilling in the ancient groove occurred around Che 444 hole in the east of 44 block. Nine wells including Che 44-51, Che 44-49 and so on are all successfully completed with oil. Meanwhile, continue drilling in the west of the ancient groove, five holes including Che 44-521, Che 44-520 hole and so on are also successfully completed with oil. Expanding reserves exploitation in Che 40 and 44 blocks, successfully completed 18 holes in Che 44-410 well field of the west inherited ancient groove and all find hydrocarbon, new oil area is 1.0Km2, new reserves are 700000 t. In Che 17 well field of the east ancient groove which is high now, nine holes are successfully completed with oil, including Che 44-60 hole and so on, oil area expand eastwardly. In the middle of Che 40 block, four holes is complete in inherited ancient groove, including Che 40-27-21 hole and so on, and also all penetrate oil layers.The results show that, superimposed fan model with its controlling factor-the"source-slope-depression arrangements"as well as the synthetic analysis of lake level changes have played an important role in this area and also received good results in practice.