AAPG ACE 2018

Datapages, Inc.Print this page

Deep-Marine Reservoir Distribution Against the Flanks of a Syn-Depositionally Active Salt Diapir

Abstract

Deep-marine successions onlapping salt diapirs can form excellent reservoirs with some combination of stratigraphic and structural trapping. However, these are typically poorly-imaged in the subsurface due to salt overgrowth, steep structural dips, fractures, and variable lithological distributions. Using an outcrop analogue is therefore a useful way to constrain facies and geometrical configurations of these complicated onlaps. The Bakio Diapir, Basque-Cantrabrian Basin, northern Spain, provides excellent exposure of a halokinetically influenced deep marine succession. The Triassic-aged gypsum body, which was growing in the Albian, is flanked and capped by Albian-Cenomanian aged deep-water deposits. The exposed stratigraphy consists of lower Albian carbonaceous marls and breccias which are overlain by distinctly different siliciclastic packages on different flanks of the diapir. To the East the Bakio Breccias are overlain by the Sollube and Cabo Matxixako units which represent thin-bedded, fine-grained siliciclastic turbidities with marls and amalgamated, thick-bedded, coarse-grained siliciclastic turbidites respectively. To the West, the Punta de Bakio and Jata units overlie the Bakio Breccias, these represent marls interbbeded with fine grained siliciclastics and marls interbedded with fine- coarse grained turbidities and slumps respectively. Within the siliciclastic strata there is evidence of extensional faulting and localised flank failure, and a complex interbedding of slides, slumps, debrites, hybrid beds and turbidites. These facies generate a complicated onlap geometry and variable rates of reservoir deterioration towards the pinch-out. Depositional models for the Cabo Matxixako unit are presented; it is interpreted as a series of stacked deep-water lobes affected by both halokinetic growth and autocyclic flow processes. Understanding reservoir deterioration towards pinch-out is critical for estimating column height and stratigraphic trapping potential; these results are applied in the development of conceptual facies models for Paleocene-aged salt-affected reservoirs in the North Sea, UK.