Facies Classification and Geophysical Signature of Lacustrine Carbonates, Case Study of a Presalt Analogue Outcrop in the Green River Formation

Abstract

Outcrop analogues play a key role in improving our understanding of subsurface reservoir architectures. The objective of this study is to characterize an outcrop analogue of the Atlantic pre-salt carbonate reservoir. The Laney Member of the Green River Fm. corresponds to marginal lacustrine carbonates including a large variety of facies, facies such as insect-microbial build-ups, tufas, coquinas, skeletal and oolithic grainstones, and silty marls. In terms of architecture, this interval is organized in patches of reef-like morphologies made up of insect microbial build-ups, with breccia and shales in between. The internal architecture of the microbialite build-ups is complex, with various nested scales of heterogeneities. To deal with this issue, we build an integrated study workflow based on a geophysical dataset acquired at different spatial and frequency scales. Thus, spectral gamma ray and sonic velocities are measured along five sedimentological logs with a fine spacing. Then, porosity and density are measured on samples collected more sparsely. Finally, 2D seismic refraction surveys are carried out at the top of the studied outcrops in order to visualize velocity interface in depth. Firstly, spectral gamma ray and sonic velocities are used to determine electrofacies using probabilistic classification methods. These facies are compared to the ones described on the outcrops. Reservoir properties are introduced into the classification by using Pore Size Distribution and Kr/Pc curves. Results of this first part is the identification of reservoir facies. Secondly, we compared acoustic signature obtained at different scales and with different methods on the outcrop. P-wave sonic velocities recorded on log-outcrop are combined with density measurements and high frequency Ricker signal to derive synthetic seismograms. Main reflector are compared to sedimentary limits observed on outcrop. Seismic refraction survey are processed in two ways: classically using first breaks to derive 2D P-wave velocity profiles and more innovative using surface waves to derive 2D S-wave velocity profiles. Sharp changes in velocities are compared to main reflectors of the synthetic seismic and to stratigraphic markers. Finally, this work gives usefull tools to characterize accurately a microbial dominated marginal lacustrine system.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018