Mechanical Stratigraphy in Cyclic Platform Carbonates, Arrow Canyon, Nevada

Abstract

For many naturally fractured reservoirs, predicting the density, distribution, and character of fractures is essential for forecasting reservoir performance and optimizing development. In some platform carbonates, natural fractures are tied to mechanical stratigraphy— the contrasts in bed thickness and strength set up through depositional and diagenetic processes— which can guide fracture prediction using sequence stratigraphic principles.

This study, a group exercise carried out during an industry field school, evaluated the tie between fractures and stratigraphy in platform carbonates in Arrow Canyon, Nevada. The Arrow Canyon Range is part of a Cretaceous thrust-related fold subsequently exposed during Neogene basin and range extension. Arrow Canyon incises through the anticline forelimb, exposing 700 m of cyclic Pennsylvanian carbonates. Dominant fracture trends strike parallel and normal to the fold hinge.

This study examined ‘type’ cycles from four intervals, chosen to capture variability in facies and their stacking: Morrowan middle-inner ramp cycles, Atokan outer-middle ramp cycles, Atokan outer-inner ramp cycles, and Missourian restricted inner ramp cycles. Facies, bed, and cycle boundaries were defined, and 8-10 Schmidt Hammer rebound values, a proxy for unconfined compressive strength, were collected in each bed. Fractures were counted along a line length of each bed and classed as intra-bed, bed-bound, intra-cycle, cycle-bound, or through-going. At the cycle-scale, fractures were classed as intra-cycle, cycle-bound, or through-going.

Hypothesized trends in mechanical behavior were subtly expressed in these rocks. The expected inverse correlation of bed thickness and fracture density was strong in Morrowan strata but weak in Atokan and Missourian strata. Only in Morrowan strata, did stronger beds have higher fracture density and did thinner cycles have more cycle-bound fractures. In both sets of Atokan cycles, the tallest fractures commonly did not propagate through the muddy outer ramp facies within the transgressive portions of cycles, indicating a stratigraphic control on fracture height at the hemicycle or systems tract scale.

This study illustrates that while stratigraphy influences fractures, multiple parameters impact fracture development and propagation. Improved correlation may come from isolating fracture sets, from more selective Schmidt Hammer data collection, and from identifying outcrops less influenced by local secondary structure.

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