3D Modeling of Fracture Density and Connectivity Within Faulted Chalk Reservoirs – A Case Study From Flamborough Head, UK

The anisotropic distribution of fracture density and connectivity in 3D across a fault zone can exert a strong control on fluid flow. However, fracture density and connectivity values are usually estimated quantitatively using 1D and 2D fracture datasets, which do not take into account the aspect ratio of the individual fractures. In this study LiDAR based fracture datasets were used to model the anisotropic distribution of fracture density and connectivity in 3D across a fault zone developed in a chalk reservoir. We modeled fracture aspect ratios ranging from 1/1 to 1/8 in order to test the control exerted by fracture dimensions on connectivity and its spatial variation. Finally results obtained from 2D and 3D dataset analyses were compared in order to give a best fit estimate on the real aspect ratio of the fractures. We studied a complex, normal fault with a total displacement of about 25 m, developed within low-porosity, fine-grained Upper Cretaceous chalk which is well exposed in the Flamborough Head area (UK). The FZ comprise of two fault cores (FC), which are both striking ENE-WSW and are offset 4-5 m from each other. Each of the FCs are up to 2 m thick, and made of fault breccias and intense veining. The damage zones (DZ), developed on both side of the FCs, contain thick (up to 15 cm) veins displaying large grain size crystals.

Detailed field-based structural observations and mesoscale data collection along 1D-, and 2D fault orthogonal transects were integrated with laser scanner technology (LiDAR). Results of 1D and 2D analyses showed that fracture density and connectivity in the DZ are about two times higher than in the FC. Within the DZ a high fracture density and connectivity domain (ICDZ) has been identified next to the FC and a high fracture density but low connectivity domain (WCDZ) further away from the FC. Based on the LiDAR data, fractures of the FZ were modeled in 3D with 5 different aspect ratios ranging from 1/1 to 1/8. The more elongated the modeled fractures were, the higher fracture density and degree of connectivity was calculated. The WCDZ and the ICDZ domains could be identified when using the lowest aspect ratios (1/3 to1/8). The 3D model results that fits best with the fracture density and connectivity values from the 1D and 2D analyses are the one with the 1/5 aspect ratio. Our model allows us to define a best fit aspect ratio that later can be used in fluid flow models to define the fluid transmissibility across the fault zone.