Paleokarst and Its Characteristics of Dengying Formation in Ziyang Area

Key words: 
• Ziyang area /
• Dervgymg Formation /
• palecdmrst /
• karst characteristics

Abstract: In Ziyang area, another gas bearing block in Sichuan Basin has been found in Sinian Dengying Formation. Dengying Formation is a set of carbonate sediments, which includes algal granula dolostone, algal cementation dolostone, micrite dolostone and crystal powder cryptomere dolostone. To the origin of the gas bearing block, there are some different explanations: 1) the result of weathering, meteoric water leaching and later tectonic process; 2) formed by recrystallization and later burial dissolution; 3) generated by dolomitization and dissolution; 4) main control factor being paleokarst. According to the study of seven drills, regional exposure characteristic and tectonic evolution history, it can be concluded that paleokarst existed in Dengying Formation in this area and it took place during Tongwan Movement at the end of Sinian. Paleokarst profile can be divided into three zones: surface, vadose and phreatic dissolution zones. In the vadose and phreatic dissolution zones, four karst forms can be found: dissolved fissure, spelaeum, dissolved hollow and cave. In this area, two karst cycles can be identified in Dengying Formation. The layer which formed by superposition of phreatic dissolution zone in first cycle and vadose zone in second cycle is the position where karst is the most developed, and the maximum vugular pore space is 42.5%. According to study, it can be found that Cambrian silicon phosphorus sediments have the characteristics of compensation sedimentation, which lie on the relic Dengsan and Dengsi section. On the basis of thickness of silicon phosphorus sediments, the paleokarst topography can be reconstructed,i.e: in south west, it is mosor high land; in north east, it is mosor bottom; between them there is a mosor slope. All topography shows from higher in south west to lower in north east. All of these provide the necessary information for the research on reservoir characteristics and forecast prospecting targets.