A Predictive Model for Karst Reservoirs in Sequence Stratigraphic Framework - a Case Study from Cambrian-Ordovician in the Tarim Basin, NW China
Tarim Basin, located in the northwest of China, is the largest petroliferous basin with Lower Paleozoic carbonate reservoirs. High heterogeneity of karst reservoirs results in a high degree of difficulty in prediction of their distribution. It is well-known that there are three kinds of paleokarst in carbonate sequences. The first occurs at the top of PACs (punctuated aggradational cycles); the second is discovered below the third-order sequence boundaries; and the third is located beneath large-scale unconformities with weathering crust. In order to search for effective predictive methods for karst reservoirs, we try to research after a predictive model for karst reservoirs’ distribution within a sequence stratigraphic framework. Based on the research results of the karstification developed in the different kinds of high frequent sequences from the outcrop sections, karst horizons recognized through the FMI and normal well logs, and sequence stratigraphic framework, together with the systematic analysis of stable carbon isotopes, we studied the karstificational characteristics developed in different types and orders of the sequences. We discovered that the multi-stage of karst horizons developed below the sequence boundaries were resulted from the periodic exposure of high frequent and third-order sea level falls. The lower the base level is, the larger the scale of the karst horizon is in a progradational sequence. The favorite areas for the karstification development in a sequence are those where the periodic exposure occurs with the periodic sea level fall, and such favorite areas for karstification are predictable. The onlap zones in the period of base level rise, the toplap and the progradational zones in the stage of high stand and gradual fall of base level are the favorite areas for the development of karst reservoirs. Based on the predictive model and seismic data, the favorite karstificational horizons and their distribution in the Cambrian-Ordovician of the Tarim basin were predicted and some horizons have been verified by the exploration.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009