Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

Abstract

Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N–S direction and the second set (Set 2) has a NE–SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter.