Ground-penetrating radar imaging of depositional and diagenetic features in an Upper Miocene carbonate reservoir analog, SE Spain

Abstract

Optimal hydrocarbon recovery from microbialite-oolite reservoirs can be limited by complex facies geometries and porosity distribution. This study integrates ground-penetrating radar (GPR) and detailed outcrop data for 3D mapping of porosity distribution and stratigraphic architecture within Upper Miocene microbialite-oolite sequences (Terminal Carbonate Complex) of southeastern Spain. Twenty kilometers of 2D and 3D GPR data of differing frequencies (25, 50, and 100 MHz) were acquired throughout the survey site, an area approximately 400 m x 300 m. Two-dimensional GPR profiles were collected along a grid covering the entire survey area, allowing imaging of depositional and diagenetic variability over a broad area. Two densely spaced 3D grids allow further detailed characterization and comparison of paleotopographic high and low areas. Results of this study illustrate that GPR can image stratigraphic features such as sequence boundaries and tight thrombolite bodies. Reflections are primarily influenced by porosity variations within the strata. Three of the GPR facies, recognized on the basis of reflection signatures (strong, weak, or no internal reflections), are linked predominantly to porosity variations in facies. Each GPR facies has been identified and delineated in each frequency data set. GPR facies distribution illustrates a large-scale trend of lower porosity in the updip portion of the survey area. GPR successfully imaged some depositional features (i.e., sequence boundaries and some thrombolites), however, in many instances, reflections correlate to variations in porosity that vary laterally and vertically on a several-meter scale in 3D and commonly cut across sequence boundaries. The abrupt lateral changes in GPR character are inconsistent with their being controlled by depositional processes; instead diagenesis and potentially fractures appear to strongly influence GPR signal. These observations indicate that porosity characteristics in the study area are associated with both depositional and diagenetic processes. Although GPR has traditionally been used to image stratigraphic features, these results indicate GPR can be important for imaging diagenetic features as well. As such, GPR can be a valuable tool for imaging porosity heterogeneity and developing conceptual models that portray 3D variations in both stratigraphy and porosity. Resultant insights may be important for comprehensive understanding of porosity development and distribution in this system and subsurface carbonate reservoir analogs.