Characteristics and Distribution of IntraPlatform Beach of the LowerMiddle Ordovician Yingshan Formation Carbonate in Tarim Basin

Key words: 
• Tarim Basin /
• intraplatform beach /
• Yingshan Formation /
• seismic respond /
• exploration
    /
•  /
•  /

Abstract: Based on the analysis of regional geology, and the discontinuity of stratum and lithology, the change of sedimentary facies, and the development of palaeouplifts, the transform stage of the regional depositiontectonics cycles changed from extension to compression began at the end of the early Ordovician in Tarim basin. The earlymiddle Ordovician Yingshan Formation platform margin began bending and downwelling which caused the deepening upward depositional cycles with middlelow energy reefbank complex along the margin. And it is favorable for the intraplatform beach development of Yingshan Formation in the western platform in Tarim basin, when the intraplatform ascending with morphologic prominence and lacking of platform margin barriers. Based on the well data, mediumhigh energy arene beaches are well developed with some of grit and oolitic beaches in intraplatform in Yingshan Formation, with 3~8 layers of multicycles deposition aggraded vertically, and lenticular or sheetlike pinchout and changed quickly laterally. The seismic response of intraplatform beaches of Yingshan Formation is different from the adjacent formation by wellseismic calibration. It displayed various mound and lenticular external shapes with lightly upper convex of the cap face and relatively even basal plane. The seismic wave of  the most intraplatform beaches can be distinguished from the adjacent formation with weak reflection or chaotic reflection of top surface, but with discontinuous and  strong reflection in case of thick mudstone cap formation, and with weak amplitude, discontinuous minor axis shape or chaotic reflection or disconnection strong wave. There are overlap and oblique reflection on the flank and thinning phenomenon of the overburden layer in the seismic section. The internal of most beaches have high frequency chaotic reflection, abnormaly strong and weak amplitude alternation, and some of them have multilayer crossing or progradation reflection which indicated multicycle beaches aggradation. On the basis of  the characteristics of seismic facies, the formation thickness and seismic attributions, prediction and identification of Yingshan Formation intraplatform can be carried out in the western Tarim basin. By the regional seismic tracing and identification, there  developed many large areas of intraplatform beaches in Yingshan Formation with blocklike shape horizontally and different size, most of the scales between 100 km2 and 300 km2, and the total area up to 12 000 km2. The intraplatform beaches mainly distributed in the southern slope of Tabei uplift, the northern slope of Tazhong uplift and the periphery of Hetihe gas field, most of which distributed along the slope of paleouplift. The Ordovician Yingshan Formation intraplatform beach, had better reservoirs with many types and wide distribution and great superimposd thickness, is favorable research aspect of potential exploration region of carbonate after the platform margin reefbank field found in Tarim basin. And more important discovery can be obtained in intraplatform beaches along the favorable accumulation slopes of palaeouplift by means of reservoirs seismic prediction to choose pay targets.