Evidence of Exposure of the Upper Cretaceous Congost Carbonate Platform and Implications for Emergent Surfaces Identification From Subsurface Data

Abstract

Understanding the nature of emergent surfaces is critical to the development and production of carbonate reservoirs as they can coincide with areas of high porosity and permeability (e.g. thief zones) or degradation of reservoir properties (flow baffles). They can also be key surfaces for field- and basin scale correlation. Nevertheless, emergent surfaces are highly variable in character. Although it is possible for karstic terrain to form during geologically short time periods, on some platforms several million years of exposure can be invisible in the succession. The Congost Platform in the Tremp Basin, Spanish Pyrenees, is Turonian-Coniacian in age. It developed basinward of the underlying Pradina Platform after a fall in global sea level and comprises two principle successions. The lowermost succession comprises coral-rudist rich skeletal grainstones interbedded within skeletal wackestone, deposited within a shallow water lagoon fringed by a platform margin shoal. A subsequent fall in sea level in the mid-late Turonian led to a further basinward shift in facies belts, forming a platform fringed by coralgal boundstones. Platform growth was terminated by emergence in the early Coniacian. The emergent surface at this upper bounding surface shows little evidence for karstification. Petrographical data indicates dissolution and cementation of precursor aragonitic shell fragments by sparry calcite cements. Under CL, most pore filling cements show a dull-non luminescent-dull-bright-dull concentric zonation, with a gradual evolution from non-ferroan to ferroan calcite. It is possible that the oldest, non ferroan calcite was precipitated from meteoric water, but it is a volumetrically minor phase. Residual porosity has been occluded by burial calcite cements. A slight depletion in d13C beneath the unconformity is the only other evidence for diagenetic modification by meteoric water. Consequently, despite a humid climate and a lengthy period of emergence, the unconformity at the top of the Congost Platform is subtle; stabilization of aragonitic grains created minor amounts of mouldic porosity, but this is not visible at a macroscale. This has implications to the correlation of carbonate platforms from subsurface data, since such surfaces will have a weak petrophysical response, and may be hard to consistently identify from rock physical data. It also implies that in equivalent subsurface settings, contrasts in petrophysical properties may not occur.

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