| There widely exists non-structural oil and gas reservoir associated with carbonate paleo-weathered crust, and the karst pore-cave-fracture formed by karstification, which have been transformed by later burial diagenesis, is the mainly reservoir space and also plays an important role in oil and gas migration.In recent years, high productive carbonate paleo-weathered crust reservoir have been constantly found in our country, such as Well Gaoshi-1 located in Gaoshiti structure, central Sichuan Basin, which had been tested a large gas flow rate in second member of Sinian Dengying Formation; Well Shuangtan-1 located in northern Sichuan Basin and Well Pengzhou-1 located in western Sichuan Basin had respectively achieved great breakthrough in natural gas exploration in the carbonate paleo-weathered crust of Maokou Formation and Leikoupo Formation,-Exploration of paleo-weathered crust reservoir in Majiagou Formation in Sulige area Ordos Basin also unceasingly obtain new breakthrough, but the distribution of reservoir presents great complexity. The major types of paleo-weathered crust reservoirs can be divided into karst cavern-fractured reservoirs and dolomite porous reservoirs, Which forming oil and gas reservoir with two prominent characteristics:firstly, complicated spatial distribution regularity with obvious heterogeneity; secondly, complex process of oil & gas migration and obscure hydrocarbon accumulation mechanism. The prediction and description of carbonate reservoirs have been well studied, while the excellent paleo-weathered crust reservoirs may not form reservoir of industrial significance. So it is important to predict hydrocarbon-enriched belts by comprehensive analysis of source rock, reservoirs and migration systems.Our research investigated different types of carbonate paleo-weathered crust reservoir developed in Tarim, Ordos and Sichuan Basin with the related discipline theories and technical methods of carbonate sedimentology & petrology, carbonate reservoir geology, structural geology, petroleum geology et al., and combined with previous research results, seismic and logging data to analyse the main controlling factors of carbonate paleo-weathered crust reservoirs including source rock, reservoirs and migration systems. Firstly, comparatively study the sedimentary environment of source rock and the source-reservoir relationship to analyse the control effect of source rock; Secondly, summarizing the different types and characteristics of carbonate paleo-weathered crust reservoirs and the main controlling factors of reservoirs development; Thirdly, exploring the prediction methods of carbonate paleo-weathered crust reservoirs in different types based on the seismic data, drilling and logging data of Ordovician Majiagou Formation in Sulige area Ordos Basin, Triassic Leikoupo Formation and Permian Maokou Formation in western Sichuan Depession; Finally, comparatively studying paleogeomorphology and the role of faults played in oil & gas migration and accumulation in different regions.According to the comprehensive study we can draw the following conclusions:(1) Extensional tectonic background is the favorable condition to develop excellent source rock. The three great craton basins (Tarim Basin, Sichuan Basin, Ordos Basin) of China were in extensional tectonic environment during the early stage of their evolution and forming rift valley, which controlling the distribution of source rock.(2) The major types of paleo-weathered crust reservoirs in the main oil & gas-bearing basins can be divided into two types:karst cavern-fractured reservoirs and dolomite porous reservoirs. The former refers to cavern-fractured reservoirs which developed in original compacted rock with low porosity and transformed by the later karstification. The latter can be defined as porous reservoirs which developed in dolomite weathering crust and based on the matrix porosity coupled with the transformation of the later dissolution process forming high quality reservoirs.(3) Exploitation of hydrocarbons from strata with paleokarst development can be challenging due to tremendous permeability variability as a result of allochthonous and autochthonous sediment fills and the development of persistent fractures throughout. Crackle breccias are highly fractured rock, with thin fractures separating the clasts and only minor displacement existing between the clasts. Mosaic breccias show more displacement than crackle breccias, but the clasts can still be fitted back together. Clast-supported chaotic breccias are composed of mixtures of clasts that have been transported vertically by collapse or laterally by fluvial or density-flow mechanisms.(4) According to the infillings, solution-enhanced fractures can be divided into two types:the overlying strata collapsed filling and the original rock collapsed filling; According to the external morphology, it can be divided into fully-penetrating solution-enhanced fracture and partially-penetrating solution-enhanced fracture. Schematic diagram of solution-enhanced fracture morphologies can be set up combining the two classification criterions.(5) Based on the comparative analysis of the factors which control the formation of paleokarst in Tarim Basin, Erdos Basin and Sichuan Basin, we have considered that the key factor controlling karst development is the ancient landform of huge uplifts and huge depressions, paleoclimate and lithology are of less importance. Based on 3D seismic data associate with regional geology background, Western Sichuan Depression DP area is favorable to form karst reservoirs in Maokou formation:The exist of the onlap point of the top of Maokou Formation indicates the exposure erosion process; Normal faults form horst-graben, causing geomorphic differences that made this area well-drained. According to the seismic data, we have found a large number of beaded abnormal seismic facies and predicted them as loess dolines and gullies. Maokou Formation in West Sichuan is taken as favorable exploration potential area where develops karst reservoir.(6) The reflection wave near the loess dolines and gullies obviously bend, so seismic curvature is a valid parameter to describe its distribution. It can be combined with the residual thickness of the strata to predict the carbonate paleo-weathered crust karst reservoirs in Maokou Formation of Western Sichuan Depression. The effective method to predict the carbonate paleo-weathered crust dolomite reservoirs is, firstly, according to the principle of sequence stratigraphy and establishing the seismic sequence framework, then optimizing seismic attributes to describe the distribution of sedimentary facies. On the basis of geomechanics, it is able to predict the favorable dolomite reservoirs distribution zone in Leikoupo formation by analyzing the fault facies. |
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