Gravitationally Induced Fracture Systems in Rimmed Flat-Topped Carbonate Platforms

Abstract

This study focusses on understanding the development of fractures in carbonate platform-type deposits. From an academic perspective, fracture formation in carbonate platforms is an important aspect of platform evolution, slope stability and the prediction of natural hazards. In the oil and gas industry, the ability to predict fracture distributions and characteristics related to flow-properties of rocks is of key importance, as a significant proportion of hydrocarbon reserves are stored in naturally fractured reservoirs. To date, not all aspects of fracture development in carbonate platforms are fully understood. In rimmed flat-topped carbonate platforms, fractures may develop in absence of tectonic stress. These fractures typically form under influence of gravitational stress that results from the flat top and highly inclined slopes inherent to these carbonate platform-type deposits. The predictability of gravitationally induced fracture distributions and characteristics lies in the analysis of the local stress distribution along the depositional profile. The generic link between fracture development and stratigraphic architecture is inherent of the sedimentation system, i.e. carbonate factory, morphology and sediment distribution. Currently, fracture studies have primarily targeted opening-mode anastomosing fractures that localise around platform rim and are oriented parallel to local platform margin trends. Several mechanisms for the formation of these fractures have been proposed and it is generally accepted that oversteepening of the platform margin and compaction of basin sediments generate these margin parallel fractures. Recently, it has come to our attention that a second type of gravitational fracture might exist across rimmed flat-topped carbonate platforms. We will discuss the occurrence of systematic, slope confined margin perpendicular fractures and suggest these may result from (1) downslope loading and/or (2) compaction over antecedent gully systems. These systems not only occur in present-day carbonate depositional systems, but also were found in various fossil examples. Future research will concentrate on modelling the boundary conditions for the formation these facture systems and the applicability to fluid migration concepts in naturally fractured reservoirs.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017