Integrated Assessment For Complicated Reservoir Of Liushagang Formation In Weixinan Depression

The research on continental oil and gas reservoirs has been more and more delicate, the exploration target of offshore oil-gas reservoir has also been the small sand body, small fault block, complicated reservoir and low permeability reservoir. So the marine oil and gas production will inevitably entered the "complicated reservoir" stage, and meanwhile the corresponding assessment methods, engineering measures and exploration measures should be designed delicately. This paper carries out a study on complicated reservoir sedimentology and integrated assessment of Liushagang formation in weixinan depression(weizhou11-4N,11-1, weizhou11-IN, weizhou11-2,) based on the reservoir forming process and3D varying pattern, established complicated reservoir assessment system and specific methods which can provide the basic rule for the design of reservoir development scheme and adjusted scheme.This dissertation have reconstructed the sequence stratigraphic framework of complicated tectonic-sedimention, and revealed the controlling factors of the complicated sedimentary system using seismic data, core data, logging data, production test and dynamic analysis of well production comprehensively based on sequence stratigraphy, sedimentology, reservoir geology, logging geology and other discipline theories. Additionally, we make a further division of the sedimentary micro-facies of the key strata, analyzes sand body vertical stacking pattern, connectivity and the plane distribution laws, establish the empirical formula between thickness and width of different sand types in different depositional system and the macroscopic heterogeneity parameters. At the same time, this dissertation also analyzes the relationship between the reservoir physical properties, effective reservoir and sedimentary micro-facies systematicly, determines favorable area, and evaluates and classifies the reservoir quantitatively using geo-statistical method. Finally this research establishes a complicated reservoir geological model in the field scale by the means of random3D geological modeling, representing reservoir structure unit, property unit and the favorable reservoir zone in three-dimensional space, which provides certain basis for the development. Research results and the main ideas can be summarized as follows: We have established the uniform sequence stratigraphic framework of Liushagang formation combining the division of oil group, change of accommodation space, division of the original reservoir of base level cycles, integrating sedimentary sequence, T-R sequence and Cross.T A high-resolution sequence stratigraphy theory, based on seismic, core, well log data. We have recognized2long term cycles,9middle term cycles,24short term cycles in the1member of Liushagang formation, and a long term cycle,5middle term cycles,24short term cycles in the3member of Liushagang formation. At the same time, the target layer is divided into5oil groups,8sets of sand bodies from bottom to top according to the distribution of well location, sand bodies and the results of sequence classification. In addition, there is a low lateral connectivity of sand body and the distribution of sand body are different in each of the oil group, what’s more, the main oil group V oil group is divided into V1and V2two layers.The stable heavy mineral assemblages in the north in the upper1member of Liushagang formation are mainly leucoxene, hematite-limonite, zircon and tourmaline from the development of sand body and the quantitative analysis of heavy mineral, which indicate the sources are mainly from the west, extending from west to east. The stable heavy mineral assemblages in the north in the lower1member of Liushagang formation are mainly tourmaline, leucoxene, zircon and hematite-limonite, and zircon, tourmaline and leucoxene in the south, which indicate the sources are mainly from the southwest, extending from west to east and east to north. The stable heavy mineral assemblages in the3member of Liushagang formation are mainly leucoxene, hematite-limonite, zircon and tourmaline, which indicate the sources are mainly from the southwest.The lithology aspects, the upper1member of Liushagang formation is major composed of fine sand, silt-finestone and siltstone, the mainly sedimentary structure includes trough cross-bedding, tabular cross-bedding, small sand grain bedding, small climb stratification, deformation bedding and horizontal bedding, mainly showing tractive current deposition, gravity deposition also exists locally, however, the middle and lower1member of Liushagang formation is major composed of glutenite, conglomeratic sandstones, middle-course sandstones, laminar fine-siltstone and mud stone, the mainly sedimentary structure includes trough cross-bedding, tabular cross-bedding, mixed bedding. On the other hand, the3member of Liushagang formation includes trough cross-bedding, tabular cross-bedding, massive bedding, bioturbation structure.The upper1member of Liushagang formation logging facies can be divided into box type, bell type, funnel-shaped type, finger type and linear type through the detail core description. The middle and lower1member of Liushagang formation logging facies can be further divided into box-funnel combination, bell-bell combination, funnel-finger combination, box-box combination and box-finger combination. The upper1member of Liushagang formation can be identified as normal delta front deposition based on the core analysis and logging facies, the delta inner-front can be further divided into braided channel, mouth bar, sheet sand, inter-channel, and the delta exo-front can be further divided into branch channel, mouth bar, distal bar, sheet sand, inter-channel. The lower1member of Liushagang formation can also be divided into delta inner-front and exo-front, further divided into8microfacies. The upper3member of Liushagang formation can be identified as delta exo-front, including4microfacies, the lower part is identified as delta inner-front including5microfacies, the delta inner-front can be further divided into braided channel, mouth bar, sheet sand, inter-channel, and the delta exo-front can be further divided into branch channel, mouth bar, distal bar, sheet sand, inter-channel.The sandstone superimposed structure is established and divided into6categories,8classes, including braided bar-braided bar pattern, braided bar-braided channel pattern, braided channel-braided channel pattern, branch channel-branch channel pattern, branch channel-mouth bar pattern, branch channel-sheet sand pattern. In the normal delta facies, the average thickness of mouth bar, distributary channel and distal sand are3.4m,3m,1.9m, respectively, the average width of mouth bar, distributary channel and distal sand are296m,294.8m,207.1, respectively, the average width-to-thickness ratio of mouth bar, distributary channel and distal sand are100.9,110.7,115.9, respectively. In the braided river delta, the average thickness of mouth bar, distributary channel are4.2m,6.3m, respectively, the average width of mouth bar, distributary channel are350.9m,348m, respectively, the average width-to-thickness ratio of mouth bar, distributary channel are84.3,70.3, respectively. In the fan-delta, the average depth of braided channel, braided bar, branch channel, mouth bar are3.6m,4.3m,4.1m,2.7m, respectively, the average width of braided channel, braided bar, branch channel, mouth bar are646.1m,783.2m,748.3m,683.9m, respectively, the average width-to-thickness ratio of braided channel, braided bar, branch channel, mouth bar are223.2,242.2,253.7,606.3, respectively.We have analyzed the changing law of physical properties. The results show that porosity ranges from12%to18%, permeability ranges from24md to294md, with regard to reservoir properties, the braid-channel and distributary channel are the best, concentrating around the wells WZ11-1-A5_WZ11-1-2_WZ11-1-A10_WZ11-1-A8. We find it that porosity and permeability in braid-channel has positive rhythm characteristics, however, the mouth bar shows inverse rhythm characteristics, the highest penneability variation coefficient concentrates around well WZ11-1N-2and WZ11-1N-A, the average permeability variation coefficient is about2.4, indicating strong heterogeneity.The log-seismic technique is introduced to track the sand distribution detailly, we find it that the normal delta deposition sand of upper1member of Liushagang formation in the seismic profile shows poor continuity, isolated or banded distribution, on the contrast, the fan-delta front sandbodies of the middle and lower1member of Liushagang formation are mainly composed of conglomerate and coarse sandstone with a large sedimentary thickness. The deposition is manifested as interbeds between thin braided channel deposition and relatively thick mudstone in the canal. There are12sand body developing in3member of Liushagang formation, showing moderate-strong amplitude, high continuity. We can conclude that the sand bodies in the north which is near the provenance are larger and fewer, however, the sand bodies in the south which is far away from the provenance are smaller and richer. We have evaluated the complicated reservoir parameters comprehensively combined with the measured data and logging data. This dissertation has established porosity interpretation model using density and neutron logging data, and established the relationship between porosity and permeability according to the grade above or below the medium sand. We have divided the flow unites into4categories using RQI and Oz combined with578test data in6wells, and established the permeability interpretation models according to the different flow units. The results show that the correlation coefficient in each permeability interpretation model is above0.8under the control of the flow units. By studying the reservoir parameters, we conclude that the good lithology of reservoir of Liushagang formation is coarse sandstone. The coarse sandstone are mainly high permeability reservoir and the fine-grade sandstone (siltstone) is mainly low permeability reservoir. The reservoir is mainly high and medium permeability in general. The lower limit of porosity and permeability are14%,2md, respectively, and upper limit of ash content (Vsh) is15%. And generally the sensitivity of Liushagang formation showed no speed-sensitive or weakly speed-sensitive, weak-acid sensitivity, weak-stress sensitivity, the speed-sensitive are mainly due to the particle size of kaolinite and pore throat match, only individual samples show strong stress-sensitivity and strong acid-sensitivity.We have proposed different testing systems according to different reservoir physical properties based on the analysis of existed complicated reservoir testing system of Liushagang formation, and compiled a set of program to calculate reasonable testing time. We have also analyzed the effect of starting pressure gradient and stress sensitive of permeability to the productivity, and proposed a reasonable method to predict the productivity of the region based on the comprehensive understanding of regional reservoir. Finally, we consider that the test method-"on-off’can best fit the low permeability reservoir of Liushagang formation, and it can achieve high yield only when the porosity is greater than17%, and specific productivity index is also increasing with the increase of permeability, so this further demonstrates that permeability is an very important factor to the productivity of the oil well, we should take a full consideration on the permeability. The relationship between productivity-buried depth and productivity-sedimentary of the complicated reservoir showed that the reservoir with abnormal high pressure has a better productivity at the same depth, distributary channel has the best property, then the mouth bar, sheet sand, distal sand, and the worst is mud.We have been clear about the geological factors affecting the complicated reservoir formation and quality, established the reservoir comprehensive evaluation system based on effective thickness, reservoir property, sedimentary facies zone, and the inter formational and internal heterogeneity, formed a comprehensive multi-parameter weighed quantitative evaluation technique. Meanwhile, the complicated reservoir of Liushagang Formation is classified, and the producing degree of reserves and recovery rate are also analyzed. Eight factors, including sandstone thickness(H), effective thickness (He), micro-facies (facies), porosity (POR), permeability(K), coefficient of variation (Vk), interlayer density (F), are introduced to classify reservoir using grey theory and grey relation method based on the actual regional situation, we divides the reservoir into Ⅰ,Ⅱ,Ⅲ,Ⅳ classes, and Ⅰ,Ⅱ,Ⅲ classes are subdivided into2subclasses, respectively. In addition, the1-1,1-2,Ⅱ-1, Ⅱ-2, Ⅲ-1are classified as conventional reservoir (a natural productivity), and Ⅲ-2, IV are unconventional reservoir in normal pressure formation. These recommendations provide technical guidance and support for the development of complicated reservoir of Liushagang Formation.We have integrated the structural modeling technique of complicated reservoir, including the "Y"-shaped fault modeling technique, the stratigraphic contact relationship processing technique, overthrust fault modeling techniques and the facies modeling technique, including reservoir inversion technique, trend surface modeling technique, well-seismic-facies technique, then the technique for complicated reservoir is established. In addition, the seismic inversion, sedimentary micro facies was used to control lithology modeling, and porosity, permeability, saturation and lithofacies simulation are more likely in accord with the actual geological features, the reserves fitting error is about7.6%, indicating that the complicated reservoir modeling technique is suitable for complicated reservoir and complicated fault block, and the accuracy of modeling is high, it can also reflect the underground structure and sand body structure more precise, and is better than the conventional modeling technique.The process, methods and technique system of complicated reservoir is established using all the methods and technique above, including geology assessment technique, logging-seismic assessment technique, integrated parameters assessment technique and3D geological assessment technique. The integrated assessment technique has been used in WZ11-4N and WZ11-1oilfield during2011-2013, and the effects of development adjustment is better.