From digital outcrops to DFN modeling of fractured platform carbonates

The use of field analogues in fractured reservoir studies is increasingly becoming popular because while large faults are mappable using geological and seismic data and small faults/fractures via well data, they are bound by certain limitations. For faults, it only provides limited information about dimensions, kinematics, and crosscutting relations with both primary and secondary heterogeneities visible at reservoir scales. More so, seismic scale data is unable to provide key information regarding fracture aperture, geometry, and overall degree of connectivity. This uncertainty hence deters working out realistic flow models, for this reason, the field analogues are used to generate digital outcrop models, bridging the gap between well log and core plug data and seismic data. The use of digital outcrop model approach to field analogues (outcrops) offers several advantages for the geoscientists. For instance, solving the inaccessibility challenges posed by some outcrops, allowing the geoscientists to better appreciate the structural architecture of diffuse and fault localised data at different scales of observation.

Our work involves the study of fractured and faulted Jurassic-Cretaceous platform carbonate rocks of the Viggiano Mt., southern Italy, which lie on the NE margin of the High Agri Valley, an intramontane Plio-Quaternary basin. We assess the geometry, distribution, kinematics of the high-angle faults, and the multiscale properties of both diffuse and fault-related fractures. The goal is to compute the transport and storage properties of the platform carbonates at outcrop-to-reservoir scale by building multiple DFN models. The outcrop scale models (50 m-side) are populated with field data and small fault data from structural interpretation of digital outcrop models. The porosity and equivalent permeability results from these models are used as matrix input for a medium size models (500 m-side) model populated with faults documented by digital outcrop analysis. The reservoir scale model (5 km-side) incorporates the latter petrophysical results as matrix input, whereas structural discontinuities are those reported in the 1:10,000 scale geological map of the study area.

Our methodology includes field data collection using linear scanline and circular scanline techniques. Data acquired digitally at late morning hours using a DJi Mavic II zoom drone with its generic camera model FC2204 (fixed focal length of 25mm, ISO -100, diaphragm opening of F/2.8, shutter speed of 1/200s), with a minimum of 280 digital images collected with >75% overlap for the 4 outcrops are processed using the Agisoft Metashape® software running on a computer with a Windows 10 OS equipped with a 64Gb Ram, an Intel core i9 (9^th generation) processor and a NVIDIA GeForce RTX 2080 graphics card (32GB dedicated Ram). Structural data were extracted using the Open plot® and Cloud Compare®, are then processed using the FracpaQ®, and statistically computed using Microsoft Excel®. The data obtained on fracture attributes are inserted into Move® to build DFN models. As a result, the values of porosity and equivalent permeability are computed for the different structural configurations/scales. Preliminary results are consistent with small discrepancies existing between results obtained by field and digital structural analyses, and scale-dependant variations of the high-angle fault network.