The Structural Characteristic Of Weathering Crust And The Evaluation Of Reservior In Ordovician At Tabamiao In North Ordos Basin

At present, the prospect of upper Paleozoic exploration and development which has proved gas reserves of two hundred and sixty-one billion five hundred and seventy-one million cubic meters has formed at Tabamiao area. So the urgent task is seeking for new substitutive strata of reserves and making an advance of exploration and development. Ordovician weathering crust has acquired some achievements of Oil and Gas. The stratum based on palaeokarst is substitutive region of reserves. According to the tectonic and geological evolution, the characteristic of the present structure and reservoir geology of Tabamiao Area, the weathering crust which Based on stratigraphy,petrology,minerageny,well-drilling and logging,well-logging,geochemistry is recognized. Combine the characteristics, types, distribution and evolution mechanism with the karst and all kinds of geology effect to analyze palaeokarst geomorphology and the dominated effect on kart development, to ascertain geomorphology typical dividation and sectional characteristics of Ordovician weathering crust at Tabamiao Area. Lever-cave sand and conglomerate are an indicator of recognizing multiple karst cycles, and a linkage between karst cycles and sedimentary sequences and cycles of sea level change. Developing of three lever-karst belts reflects that lever-cave sand and conglomerate occur stratum, thickness and depth of a well at Tabamiao. and according to the characteristics of three lever-karst belt, we can ascertain not only the distribution but also the contrast, and confirm the differences and causes of three lever-karst belts. Through the analysis of tested well logs' characteristics, the fast identifial mode of palaeokarst reservoir. According to well logs, physical property of wathering crust , we have founded gas stratum evolution criterion and ascertained that it belong to poor porosity and poor permeability weathering crust reservoir. We do not only evaluate four characteristics of reservoir, reservoir characteristics of palaeokarst geomorphology units, distribution of reservoir characteristics comprehensively, but also predict advantageous region of palaeokarst.