Indirect analysis of unconventional shale hydrocarbon resources in the Recôncavo Basin, Brazil. A case study

Abstract

The Recôncavo basin is a mature basin with great potential for extracting hydrocarbons from conventional and unconventional reserves. It is inserted in an extensive taphrogenic basin, that is the Recôncavo-Tucano-Jatobá aborted rift in the Brazilian northeast region, where the beginning occurred with the rupture of the western Gondwana paleocontinent. Stratigraphically, it presents six sedimentary sequences related to the pre-rift and rift stages. The study area is located in the structural low named Camaçari Low, in the Southern compartment of the basin, what it is the deepest depocenter, with a preserved sedimentary section of 6900 m, it is in addition to being main oil and gas generation kitchen. So to evaluate the potential abundance of a rock formation, in the case of both conventional and unconventional reserves, it is of key importance to quantify the total organic content (TOC), hydrogen index (HI), vitrinite reflectance (Ro), maximum hydrocarbon generation temperature (Tmax) and level of organic maturation (LOM). These parameters can be estimated by direct methods (in the laboratory) using geochemical analysis or by indirect methods using well log data. Direct analysis provides point estimates, while indirect analysis provides a continuous depth profile of them. For this study, data from geochemical analysis and logging of well 1 CGL 1D BA from the Recôncavo basin, in the Camaçari Low, to estimate the TOC profiles by means of simple linear regression, LOM by the Passey method utilizing the ΔDT, Ro, HI and Tmax, for depths greater than 1000 m. The linear regression resulted in a moderately positive correlation of the TOC estimate. Replacing the ΔLogR method of Passey for ΔDT method of Bowman proved to be a rapid and easily determined technique. The estimates of the vitrinite reflectance and hydrogen index based on the estimated LOM values were compatible with the measured values, as was the estimate of maximum hydrocarbon generation temperature based on the vitrinite reflectance. The organic matter was characterized as type I and II kerogen.