Two Procedures for Stochastic Simulation of Vuggy Formations

Abstract

Abstract Simulated annealing and stochastic simulation based on indicator kriging are used to describe vuggy formations in a Mexican offshore field. Besides heterogeneity associated to these carbonated formations, vug spatial distribution is an important issue since hydrocarbon storage and deliverability to the wells is highly controlled by vugs density and connectivity. From computed tomography images, exhaustive 3D statistics are derived to build 3D stochastic models of those vuggy formations. Multiple point statistics simulated annealing and sequential indicator simulation based on kriging are used to model vugs geometry. The results indicate that both approaches honor the target statistics, and visual inspection of such images confirm good reproduction of connectivity. Further, it is shown that computer performances are different with simulated annealing being the most demanding. Although stochastic images from indicator kriging can be used as initial images for simulated annealing, it is demonstrated that few value is added by imposing multipoint statistics in the framework of vugs modeling. Therefore, it seems that stochastic simulation based on indicator kriging can build, with an acceptable computer cost and good reproduction, 3D images of vuggy formations. Introduction Oil reservoirs associated with carbonated rocks are of great economic importance because between one-third and one-half of the world oil production comes from limestone or dolomite reservoirs (Pettijhon, E.J. 1975). The offshore reservoirs located in the southeast of the Gulf of Mexico are the case; this area contains about 50 percent of the oil reserves in Mexico (PEMEX Exploración y Producción, 1999). These reservoirs are associated to fractured and vuggy carbonated rocks whose main characteristics are the rapid variation of rock quality and significant secondary porosity. These items should be addressed in any geological-petrophysical model providing insights about properties such as porosity and permeability. Under this context, vug connectivity and porous space associated to vugs and fractures can be accomplished by investigating the spatial distribution of secondary porosity from computed tomography (CT) images of cores. Therefore, the goal will be to reproduce quantitative aspects of the porous-vugular media through stochastic simulation. Geological setting The data studied come from the Abkatun field, located in the southeast part of the Gulf of Mexico (Figure 1). The main reservoir rocks are dolomitic carbonated breccias from the Upper Cretaceous-Lower Paleocene (K-T). They consist of 50 to 150 meters of coarse-grained carbonated breccias strongly dolomitized and naturally fractured with secondary vuggy porosity due to diagenetic dissolution processes, having a porosity range from 6 to 14 percent and permeabilities from 800 to 3,000 milidarcies (PEMEX Exploración y Producción, 1999). The reservoir breccias and corresponding seal origin (bentonic shale and siltstone beds), may be due to tectonic activity which created a debris flow deposited as carbonated breccias along the shelf margin and the shelf slope. Recent research relates the origin of these rocks to the K-T meteoric impact event known as Chicxulub. So the breccias sequences are likely to have resulted from a major slumping of the carbonated platform margin triggered by the Chicxulub impact (Grajales-Nishimura, et al, 2000).