Controlling Effects of Paleogeomorphy on Limestone Gas Reservoir of Marine–continent Transitional Facies

Abstract

In recent years, great progress of limestone gas exploration had been made in the Taiyuan Formation of marine–continent transitional facies, eastern Ordos Basin. Taking the Taiyuan limestone gas reservoir as an example, paleogeomorphic characteristics were studied and restored by moldic method, and the controlling effects of paleogeomorphy on the reservoir were mostly analyzed. The results showed that: (1) The sedimentary paleogeomorphy was mainly high in the west and low in the east, with alternating distribution of local highs and depressions, and the height difference ranged from 20 to 50 m. High yield wells, such as Yu 20, Zhou 2 and Zhou 3, were obviously at local highs of the paleogeomorphy; (2) Paleogeomorphy had strong promoting effects on the reservoir development. On the one hand, the local highs were favorable for limestone deposition when water body was deep. On the other hand, the deposited limestone of local highs was easy to be exposed and leached by atmospheric fresh water, which promoted the formation of dissolved pores and fractures. With good physical conditions, the local highs of paleogeomorphy were golden area for reservoir development. (3) Paleogeomorphy largely controlled the development of source rock. In local highs of the paleogeomorphy, it was conducive to sustainable development of peat swamp and forming thick mudstones or coal seams when the water body was shallow, which provided good hydrocarbon source and sealing conditions for gas accumulation. (4) Paleogeomorphy highly dominated the spatial matching of source and reservoir. With the change of water depth, the limestone, mudstone and coal seam were deposited interactively. The interactive deposition formed an interbeded combination of source rock and reservoir space, which was conducive to forming of gas reservoir in source.