Assessing the Impacts of Geopressure on Exploration Using Integrated Geological Log Analysis

Abstract

The occurrence of geopressure anomalies in the El Furrial, Chaguaramal and Boqueron oil fields in the Maturin subbasin of the Eastern Venezuela Basin, and their heterogeneous evolution through geological time has caused significant drilling problems. Identification of these anomalies and isolation of the adjacent subcompacted formations has cost much time and money. There are two main abnormal pressure zones in the Eastern Venezuela Basin. The lower occurs in a thick homogenous marine shale sequence of the Carapita Formation, deposited during late Oligocene-early Miocene times, which was followed by a late Miocene period of uplift and erosion. The upper occurs in marine shales and sandstones of the La Pica Formation, which was transgressively deposited on the eroded Carapita Formation. Using an integrated analysis of lithology and wireline logs, tectonically-influenced sections have been identified within the lower geopressure which have modified its original conditions. Furthermore, the late Miocene unconformity between the two zones controls the occurrence, style and dimension of the upper geopressure. Detailed differential compaction analysis from low to high hydraulic potential intervals through the lithostratigraphic sequence and their association to the unconformity explain the evolution and the current state of geopressure distribution throughout the oil fields. These conclusions are supportedmore » by direct and indirect quantitative pore and fracture pressure gradient analysis. The study identifies areas within these fields (e.g., Central and Eastern Boqueron) which show alterations of the occurrence and behavior of the geopressure. Such knowledge permits rapid modifications to drilling programs, allowing successful exploration and development.« less