Introduction to Karst Systems and Paleokarst Reservoirs
Mateu Esteban, James Lee Wilson, 1993. "Introduction to Karst Systems and Paleokarst Reservoirs", Paleokarst Related Hydrocarbon Reservoirs, Richard D. Fritz, James L. Wilson, Donald A. Yurewicz
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Karst is the product of subaerial (terrestrial and coastal) exposure of carbonate rocks, recognizable by features produced by dissolution, precipitation, erosion, sedimentation and collapse in a variety of surface and subsurface landforms, and cave deposits consisting of both cements and sediments. Natural karst constitutes a drainage unit (Fig. 1) consisting of: (1) input of meteoric waters, (2) pre-existing permeability pathways enhanced or reduced by karst flow, and (3) output of resurgent waters with transported sediments and solute. There are various types of karst depending on rock types, insurgence and flow patterns, climate, and etc., corresponding to different modes of porosity creation and destruction. Lithologies can be: (a) tight (dense) with bedding plane control, (b) tight, with fracture control, and (c) porous, with intergranular porosity control. Flow patterns can be diffuse, confluent, allogenic (water collected from non-karst drainage) or authigenic (catchment surface is karst) (Fig. 2).
Most diagenetic models for subaerial exposure in carbonate rocks have been developed in Holocene Caribbean carbonates, forming a karst in very porous carbonates with diffuse recharge and flow, short exposure times, low relief and interaction with coastal exposure environments. This is but one of the many types of karst and these diagenetic models cannot be applied in many of the cases encountered by explorationist.
Karst systems present zoned flow patterns, normally with many anomalies in the distribution of the hydraulic potential (hydraulic traps, confined flow) and resulting thermal and chemical zonation The level of regional groundwater saturation (water table, piezometric level) separates the infiltration or
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This volume is a compilation of papers relative to paleokarst and associated reservoirs. The examples illustrate many of the rock types, and stratigraphic, structural, and paleotopographic features of carbonate strata which result chiefly from solution and collapse due to ingress of meteoric waters at and below unconformities. Examples presented here range from settings with considerable dissolution and collapse to those with significant unconformities but little evidence of meteoric alteration. It is estimated that 20–30% of recoverable hydrocarbons are in some way related to unconformities. Paleokarst reservoirs may also be important future reservoirs for application of horizontal drilling technology.