| Deep-burial karst reservoirs in carbonate rocks are important petroleum exploration targets in the world. An extensive unconformity was created on the top of Middle-Lower Ordovician in Tarim Basin. As a result, typical karst reservoirs were produced in these formations and lately became significant petroleum-bearing intervals. However, the Middle-Lower Ordovician had underwent extremely complex tectonic evolution history and multiple-phases’ diagenesis fluid activities. Therefore, the architecture and development characteristics of reservoirs tend to be complex, and also the karstification types and reservoirs formation mechanism have not yet been well documented and remain controversial. This situation restricts the further processes of petroleum exploration and exploitation. An unconformity was formed on the top of Ellenburger group in Lower Ordovician in Permian Basin, west Texas, which indicates that Ellenburger group holds similar karst geological condition with Middle-Lower Ordovician in Tarim Basin. However, the architecture and distribution laws of reservoir spaces in two basins vary a lot. To analyze the differences between them will be critical for the recognization of karst reservoirs formation mechanism in Middle-Lower Ordovician in Tarim Basin.The Middle-Lower Ordovician in Tazhong Uplift, Tabei Uplift in Tarim Basin and Central Basin Platform in Permian Basin were selected to be studied in this dissertation. We utilized the theories of karst hydrology and geomorphology, hydrogeology, reservoir geology, seismic geomorphology and logging geology to carry out this study. Additionally, the combination of geology and geophysics and geochemistry was emphasized when we conducted the analysis about reservoir development features both in macroscopic and microcosmic scales. Then the types, architecture, features and distribution characteristics of karst reservoir space and the fundamental control factors of them were displayed. Later, the porosity types, the diagenetic fluids types and origin and the porosity formation mechanism were presented through the diagenesis evolution analysis. Furthermore, the types, phases and scale of karstification which contributed to reservoirs production and preservation were discussed. Besides, the hydrogeological models of reservoirs were established under the guidance of karst theories about hydrogeomorphology. Finally, we demonstrated the differences about reservoirs development features and formation mechanism in Middle-Lower Ordovician in Tazhong Uplift, Tabei Uplift in Tarim Basin and Central Basin Platform in Permian Basin and concluded the reasons of thus differences. Several conclusions were documented as followings:1. Fractures, vugs and isolated caves consist of the reservoir spaces in Middle-Lower Ordovician in Tazhong Uplift, which are overprinted by mixing water in eogenetic diagenesis environment, unconfined meteoric water in subaerial diagenesis environment and ascending thermal water in burial diagenesis environment. Consequently, three different categories of karstification came into being. The contribution of mixing karst to reservoir formation is restricted and the origin of reservoirs creation is the combination of unconfined meteoric karst and hypogenic karst.2. The diagenesis environment was identified to be eogenetic environment when the Middle-Lower Ordovician was uplifted to surface in northwestern Tazhong Uplift, according to the tectonic evolution history analysis. Therefore, we recognized two types of karstification including mixing karst and meteoric karst in eogenetic diagenetic period. They occurred in the depositional period and uplift period of Middle-Lower Ordovician respectively.3. The type of hydrogeomorphologic architecture in Middle-Lower Ordovician in Tazhong Uplift during the first phase of Middle Caledonian was determined to be non-surface drainage system category based on the theory of karst geomorphology and hydrogeology. And a "Fissure-Seepage" and a "bedding-parallel-runoff’development pattern of meteoric karst were produced under thus hydrogeomorphologic architecture in central uplift part and north-slope part respectively in Tazhong area.4. The secondary geomorphic units including karst plateau, karst slope and karst basin were divided along the direction of NE-SW in Middle-Lower Ordovician in Tahe area in Tabei Uplift. And also the surface and subsurface drainage system were recognized in this area. Therefore, the type of hydrogeomorphologic architecture can be identified to be surface-subsurface drainage system category and the karst evolution period has become senior.5. An integrated conduit system was produced by meteoric karstification in early Hercynian in Middle-Lower Ordovician in Tahe area in Tabei Uplift and 5 types of paleocave facies were recognized in this system. The morphology of paleocave system includes ramiform maze cave and branchwork passages. And we found multi-layer and multi-phase cave levels in vertical direction. This integrated conduit system is related to structural architecture, fault system, base level and hydrogeomorphologic architecture. Finally, we established a "Syncline concentration flow" and a "Canyon" karstification development hydrogeological model in karst plateau and karst slope respectively.6. A coalesced collapsed-paleocave reservoir developed in Ellenburger group in Permian Basin, and we recognized 9 types of paleocave facies. This paleocave system underwent three periods including subaerial exposure, collapse both in shallow-burial and deep-burial environment. The unconfined meteoric water related to subaerial exposure and hydrothermal water associated to Ouachita Orogeny overprinted the collapsed paleocave system. And the multiple-phases’ collapse and tectonic processes are the main origins of paleocave in Ellenburger.7. The difference about reservoirs development features in Middle-Lower Ordovician between Tazhong Uplift and Permian Basin is that a combination of fractures, vugs and isolated caves is produced in Tazhong Uplift, while a collapsed-paleocave reservoir is formed in Permian Basin. The origins of this difference cover four aspects:1) the preservation of porosity generated by meteoric water is different; 2) the types, phases and scale of dolomitization vary in two areas, which influences the preservation of reservoir spaces; 3) the multiple-phases’ collapse and tectonic processes did not occurre in Tazhong Uplift; and 4) the types of hydrothermal water in Tazhong Uplift are more than that in Permian Basin, thus the hypogenic karst developed much better in Middle-Lower Ordovician in Tazhong Uplift.8. The difference about reservoirs development features in Middle-Lower Ordovician between Tabei Uplift and Permian Basin is that an integrated conduit system is generated in Tahe area in Tabei Uplift, while a collapsed-paleocave reservoir is created in Permian Basin. The origins of this difference contain four aspects:1) the time and scale of subaerial meteoric karstification vary in thus areas; 2) two different paleocave system are formed due to different hydrogeomorphologic architecture; 3) the scale and effects of collapse in two areas are different; and 4) the origin of hydrothermal water may differ in two areas, as a result, the hypogenic karst reservoirs hold different features.9. The scale of reservoirs in Middle-Lower Ordovician in Central uplift part is larger than that in north-slope part in Tazhong Uplift. The origins of this difference include hydrogeological conditions, the distribution of pinch-out line of Upper Yingshan Formation, attitude of stratum, thickness of bedding and tectonic processes. Thus factors above generated several karst effects in Middle-Lower Ordovician in Tazhong Uplift such as hydrogeological effect, pinch-out line effect, karst geological boundary effect and tectonic related karst effect. |
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