| Qianjiang Formation of Qianjiang Depression is composed of clastic rocks, carbonate rocks and salt rock. The strata deposited in inland salt lake environment under brackish water or salt water conditions, its deposition mainly includes sandstone&mudstone interbed and salt rhythms which developed a total of193. Controlled by northern unidirectional material source, Qianjiang Formation of Qianjiang Depression mainly developed steep slope fan, delta, beach bar and salt lake facies sedimentary system,overall its northern part has a characteristic of "half-basin-wide sand-Bearing". Qianjiang Depression is the richest Hydrocarbon-rich Depressions of oil and gas in Jianghan Basin.The exploration results show that all layers of Qianjiang Formation have a characteristic of oil-bearing, among which Eq31, Eq32, Eq41and Eq43are the most enriched layers. In research area, the sandstone reservoir is thin and the lateral variation of sand body is quick; meanwhile due to the existence of sault rhythm,there form Multiple sets of wave impedance interface and multiple reflection characteristics of sault rock, sandsone, mudstone and so on, bringing comparatively large difficulties to the recognition of lithological reservoir by using seismic data. At the same time, there has already formed a complete theoretical system for the influencing factors and depositional system types of continental lacustrine basin depositional system at home and abroad, but the influencing factors and distibution rules of salt lake basin depositional system are still in continuous research stage, making it difficult to grasp the formation mechanisms and distibution rules of salt lake basin lithology reservoir. This paper takes Jianghan Basin, a typical salt lake basin, as a research object, using facies-controlled reservoir modeling to predict sandbody distribution characteristics in order to provide guidance for the exploration and development of Qianjiang Sag, Jianghan Basin, and meanwhile lay foundation for sandbody prediction of similar salt lake basin at home and abroad.Aiming at salt rhythm development, various lithology types and multiple lithology associations of salt lake deposit in Qianjiang Depression, this paper based on Seismic attribute analysis and reservoir prediction theory takes Qianjiang Sag of Jianghan Basin as a research object, using facies-controlled reservoir modeling by combination of seismic and geology to accomplish reservoir prediction of Qianjiang Formation, Qianjiang Depression. The exact research include:divide sequence stratigraphic units of Qianjiang Formation in research area, and build fine isochronous stratigraphic framework by core observation, well logs characteristics analysis and geochemical indicators characteristics analysis; summarise sedimentary facies types and characteristics of target layer in research area, analyze Sedimentary facies distribution characteristics and evolution laws within the sequence stratigraphic framework by fine core description and log facies analysis; find reservoir prediction sensitive parameters with wells and seismic data, and pick preferred reservoir prediction method according to petrophysical characteristic analysis; analyze Seismic response characteristics of sandbody with different lithology associations and genesis by forward modeling, as a basis for dividing favorable seismic facies belts, conducting favorable facies constrained seismic attributes analysis and seismic inversion; at last according to structure and deposition research results, we delineate reservoir with favorable sedimentary facies belts developed, find and recognise lithology traps combining facies-controlled reservoir prediction results.The study mainly adopts seismic attribute analysis, wave impedance inversion, density and gamma parameter inversion and pre-stack elastic parameter inversion to conduct reservoir prediction. This paper analyzes reservoir response characteristics of different attributes, and pick preferred sensitive attributes for prediction. In this way, we eliminate attributes which cannot truly reflect reservoir information, save real information, and pick parameters according to correlation between various parameters so that we can reduce information redundancy in parameter space. Through analysis we find that the maximum amplitude and instantaneous frequency attributes can better depict thick reservoir distribution. This study also evaluates and picks different inversion methods. The analysis shows:Absolute acoustic impedance inversion is suitable for a large work area and thick san dbody forecast, correlates well with seismic attributes; stochastic simulation and nonlinear constrained inversion are suitable for thin layer forecast, nonlinear constrained inversion has lower requirements for well-control degree and much bigger application scope, meanwhile multiplicity problems increase with the improvement of resolution; the structure-logging nonlinear inversion combine advantages of logging constrained inversion and modeling inversion, it not only solves the slection problem of low-frequency trends in single well constrained inversion, but also overcome the problem of given initial model in modeling inversion, and the application of nonlinear constrained inversion algorithm provide higher resolution, its prediction results will not change a lot whether well datas participate in computing or not and have strongger predictability for thin layers; the pre-stack inversion suits for complex lithology recognition and prediction, it uses the most abundant information bringing the increase of influencing factors at the same time, for example, offset of seismic data acquisition, prestack signal-to-noise ratio, fidelity and lack of full wave logging all can influence the inversion effects directly.This paper mainly acquired achievements below:1. Applying drilling coring datas, logging datas, lithology associations, seismic wave reflection characteristics signsand other auxiliary signs to recognise base-level cycle interface and discuss recognition method of salt lake basin sequence interface. On the base of comprehensive analysis, we divide Qianjiang Formation into one second-order sequence and eight third-order sequences, and each third-order sequence is divided into2-24short-term base-level cycles. Thus we build sequence modes of different units like northern steep slope zone and southern gentle slope zone in Qianjiang Depression.2. Through the analysis of sequence evolution characteristics, we think that in the earlier period of basin development, the Qianbei Fault in northern depression activated strongly, the basin bottom was northwestwardly half-graben and presented to be low in the north and high in the south, the lake basin development in the fault-depression stage was affected by paleostructure and paleogeographic background, the distribution characteristics of sequence and its internal system tracts vary a lot between the north-south and east-west, and so are the distribution and sequence patterns of lowstand system tract. In the medium-term stage of basin development, the northern boundary faults had been activating continuously, but its intensity gradually decreased, the structure evolution made the general trend of the depression paleotopography to be "north-low and south-high", under this paleogeographic background, in north-south trending, controlled by faults and provenance each system tract of the northern part was breaked by faults and the formation thickness decreased northward, in north fault slope zone there mainly developed sandstone and mudstone interbed deposition, and in depocenter there developed thick layer of salt rock and mudstone, along with thin sandstone and mudstone interbed, thus the seismic reflection was strengthened in compare with lower sequence and the reflection was relatively continuous. In the late period of basin development, the basin sedimentary magnitude diminished as the basin uplifted integrally, the whole lake basin had already become very flat, and in the traverse direction the changes of sequence thickness diminished obviously inayompare with the lower sequence. The concentration degree of the lake basin water had already reached the maximum degree, in the salt lake center there mainly developed large sets of thick layer salt rock, and due to the very defference with lower sequence lithology there maily developed fine material on the edge of the basin.3. According to the sedimentation-structure background differences of sequence development, the sequence structure within the Qianjiang Formation can be divided into two development models:fault break zone and bending zone. In the northern depression, influenced by faults, the formation presents to be missing and mainly belongs to the sequence development model of fault slope break zone; in the western slope of depression, it is characterized by a series of stratigraphic overlap and mainly belongs to the the sequence development model of bending zone. From the sequence development time, we can see that the early SⅢ1～ⅢS5sequence mainly belongs to fault slope break zone mode and the late SⅢ6～SⅢ10sequence mainly belongs to bending zone mode. The sequence model of fault slope break zone present to be single-step-fault. The down side of the fault slope break zone, on the one hand, is the deposition area of low stand system tract, on the other hand, it also limits the deposition boundary range of each system tract, the steep slope low stand fans and high stand fan delta developing in this area connect together laterally like a skirt shape,and due to the steep slope, the facies zone becomes narrow and changes quickly, the delta front sandbody often directly inserts into salt lake, and the sandstone directly change into salt rock in facies. As the northern fault slope break zone is steep and develops faults,and the sudden intermittent supply of strong sedimentary source in near-source area, there develops steep slope fan delta deposits in high and low system tract within the fault slope break zone, however, in lake transgression system tract there commonly develops gravity flow sandbody. The sequence development model of bending zone appears to be the big changes of sedimentary thickness of bending zone, commonly, in bending zone, the slope is mild, each sequence has thinner thickness and changes stably, the sedimentary cycle obviously decreases, the low stand system tract generally westward overlaps and pinches out in bending zone; but under the bending zone, the slope suddenly increases, the thickness of each sequence obviously increases and so are the sedimentary cycles, there completely develops low stand, lake transgression and high stand system tract; overall the bending model presents to be a fan shape spreading out eastward in stratigraphic framework.4. In Cretaceous period, the paleostructure of uplift and sag interval change, which was formed in the north of Qianjiang Depression, controlled the entrance of north provenance of Qinjiang Formation. The triple section structure of the Qianbei Fault and its saltative fault section caused multiple tectonic transformation zones, these zones were the main entrance of north provenance of Qianjiang Formation, Qianjiang Depression. The pre-existing regional tectonic framework and the Qianbei Fault controlled provenance entrance and scale of Qianjiang Formation. Under the control, in research area there mainly developed salt lake shallow-water delta——freshwater-brackish lake——salt lake depositional system. The distribution of delta front subfacies and salt lake facies presented to be a trade-off relationship, vertically, the general appearance was:from the bottom to top, the lake basin gradually became shallow, the sandbody developed, however, influenced by provenance supply and aqueous media condition, the deposition of each section within Qianjiang Formation had cyclicity which affected sandstone development.5. Not all salt rock deposition of Qianjiang Formation salt lake are typical "SS" evaporite formation, it is composed of salt rock with multiple genetic types. The lower cycle of Qianjiang Formation is mainly a kind of salification environment under continuous water condition with high salinity, it reflects the characteristics of deep-water salt rock in the period of high sedimentation velocity and recharge rate. The upper cycle of Qianjiang Formation, along with the overlying Jinhezhen Formation and Guanghuasi Formation are mainly the deposition of when the basin was dry up or nearly dry up.6. Through summarizing the variation regularity of different petrophysical parameters(acoustic velocity, density, wave impedance and gamma), we think, in terms of velocity, the velocity of gypsum mudstone is the biggest, the velocities of other rocks stack with each other seriously and distinguishes from each other badly; in terms of density, psalt rock<ppermeable sandstone<pmudstone, gypsum mudstone, dry sandstone, permeable sandstone can distinguish well from other rocks; in terms of wave impedance, Zsalt rock<Zpermeable sandstone<Zmudstone<Zdry sandstone<Zgypsum mudstone, the values of different lithological rocks stack with each other seriously, permeable sandstone stacks seriously with salt rock but distinguishes well from other rocks; in terms of gamma, Grsalt rock<Grpermeable sandstone、 Grdry sandstone<GRgypsum mudstone<GRmudstone, sandstone distinguishes well from other rocks, but the gamma of dry sandstone stacks completely with the gamma of permeable sandstone and cannot be distinguished. Thus, we come to a conclusion that using wave impedance to predict permeable sandstone in sandstone-mudstone formation is of multiple solutions. It’s easy to mix them up as salt rock and permeable sandstone all have lower value of medium impedance, only if we exclude salt rock can we guarantee the correctness of proceeding reservoir prediction by impedance.7. Through the application of seismic attribute analysis, seismic facies analysis, seismic inversion and time-frequency analysis, we proceeded reservoir prediction of main subject resevoir. Combining with paleostructure aspect, sedimentary micro-facies and sublayer datas of drilled wells, we predicted the reservoir plane distribution rules of Eq341, and again interpreted and researched the sedimentary micro-facies types of Guang38-6well.8. Based on the establishment of seismic prediction model, we apply methods like,seismic attribute analysis and seismic facies analysis to clearly depict thick layer sandstone. We apply attribute analysis and seismic facies analysis to recognise thick sandstone and possible thin sandstone; in the lithofacies belts divided roughly by geology, we pick up elastic impedance parameters which can distinguish lithology to recognise thin layer sandstone by applying spectral imaging analysis, post-stack wave impedance inversion, post-stack density inversion and pre-stack inversion.9. Through summarizing the lithological trap formation geological conditions in research area, we think that the present structure framework and sedimentary system characteristics of Qianjiang Formation had created favorable conditions for the formation and development of lithological trap, and thus leaded to the formation of various types of lithological trap. Based on the summary of lithological trap development characteristics, we establish the lithological trap development pattern of this area, predict the favorable lithological trap development zones and evaluate important lithological traps.The research showed that we successfully predicted the reservoir distribution characteristics of Guangbei river channel, debouch bar and beach bar by facies-controlled reservoir prediction in Qianjiang Depression, and finded out a number of lithological traps and structral-lithological traps, thereamong the lithological trap drilled by Guang38-6acquired high-yield industrial oil and got an good application effect. |
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