Multi-phase Karst Development Characteristics in the Ordovician of the Jizhong Depression

Key words: 
• Jizhong Depression /
• Ordovician /
• karst /
• distribution regularity /
• paleogeomorphology

Abstract: Ordovician karst reservoirs in the Jizhong Depression are well developed,but the development of vugs, caverns and channels varies greatly in each stratum and in each area,indicating different main karst controlling factors. At least three phases of fracture occurrence and cutting relationship were identified by microscopy analysis and at the outcrop. The study suggests that the phases correspond to the rock rupture events of the Indosinian,Yanshan and Himalayan movements. The outcrop and microscopy study of the shape of the pores,and the cathodoluminescence of the cement,indicated that the Jizhong Depression developed syngenetic karsts,burial karsts and epigenetic karsts during the Ordovician. The present karst reservoirs in the Ordovician were formed by multi-phase karstification. The tectonic activities of different periods resulted in different forms of karstification in the Jizhong Depression:the strongest near-surface tectonic activity resulted in the most rapid development of dissolution pores. A comparison of karst paleogeomorphology data and drilling breaks and mud losses in the Jizhong Depression shows that karst depressions are a favorable paleogeomorphological type. It is considered that such karst depressions were non-water storage structures. The karst highlands,slopes and depressions have horizontal karst zones of different vertical depth,which correspond to the vadose and phreatic zones that existed in different paleogeomorphologies. Large numbers of drilling breaks and mud losses occur near deep fault zones,and the homogenization temperature of fluid inclusions indicates the existence of a hydrothermal fluid. Thin interbedded sections of dolomite and limestone are favorable karst-development sites in areas with weak karstification and a low development rate of dissolution pores.