--> --> Facies classification in lacustrine carbonates derived from geophysical data, comparison between outcrop and core measurements

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Facies classification in lacustrine carbonates derived from geophysical data, comparison between outcrop and core measurements

Abstract

Subsurface subsalt reservoirs are very complex in terms of architecture and facies heterogeneity distribution. The objective of this study is to characterize through an integrated approach two analoguesof the Atlantic pre-salt carbonate reservoirs for a better understanding and documentation of marginal lacustrine subsalt reservoirs. Two formations are investigated as presalt carbonate reservoir analogues: the Green River fm. (Wyoming, USA) and the Yacoraite fm. (Argentina). The idea is to compare the results of electrofacies classification derived from data acquired (sedimentological, petrophysical and petroacoustic data) on an outcrop for the Green River fm. and on cores extracted from wells for the Yacoraite one. The proposed workflow follows identical steps for both study cases. Firstly, accurate sedimentological observations were made to recognize representative sedimentary facies and qualitatively assess main reservoir characteristics. Then, a set of geophysical data is finely acquired along the sections: spectral gamma ray (Potassium, Uranium and Thorium components) and sonic velocities (P-wave and S-wave using 250 kHz sensors). Spectral gamma ray and sonic velocities are used to determine electrofacies using probabilistic classification methods. After integration of sedimentological facies and electrofacies, petrophysical properties are then introduced into the classification by using the Pore Size Distribution and Kr/Pc curves provided by MICP (Mercury Injection Capillary Pressure) analyses carried out on small samples taken along the studied sections. This first part of the workflow allows the identification of reservoir facies (or rock-type?), populated in porosity and permeability. In a second step, we compare the acoustic signature obtained at the fine scale (by computing synthetic seismograms from P-wave sonic velocities recorded on sections and density measurements on corresponding samples) and seismic data acquired at the field scale (refraction seismic for the Green River fm. and reflection seismic for the Yacoraite fm.). We then discuss the observed similarities and differences considering both scale and frequency effects. At the end, acoustic signatures and results of facies classification are integrated to define exhaustive rock types including petrophysical, sedimentological and geophysical properties by specific carbonate lacustrine facies. The exhaustive database acquired on different pre-salt analogue formations permits us to accurately characterize marginal lacustrine systems and evaluate the impact of multiscale heterogeneity on facies distribution and reservoir properties.