Multi-Scale Characterization and Modelling of Non-Marine Microbial Carbonates: The Lower Purbeck Formation (Late Jurassic to Early Cretaceous), Dorset, Southern England (UK)

Abstract

Abstract

The discovery of Lower Cretaceous, non-marine carbonate hydrocarbon reservoirs in the South Atlantic has triggered much research into understanding how such complex strata form. Sedimentary facies and basin architecture models are few and limited in their predictive value. This study is develops new sedimentary and stratigraphic forward models (SFM) for non-marine microbialites in a semi-arid climate setting in an extensional basin based on the Purbeck Formation (Upper Jurassic – Lower Cretaceous) exposed in Dorset (southern England, UK). These are considered as partial analogues for some aspects of the South Atlantic pre-salt carbonate reservoirs.

Outcrop studies coupled with subsurface, petrographic and petrophysical studies aim to constrain and improve published models of depositional settings. Seismic and well data indicates that accumulation of strata and facies distribution of the Purbeck limestones occurred in half-grabens controlled by east-west extensional faults and an intervening relay ramp. Carbo-CAT is a SFM that was developed to assess this tectono-sedimentary model and to improve the predictability by testing a variety of scenarios of fault activity and carbonate production.

The lower Purbeck limestones are characterised by metre-decametre sized, in-situ, highly porous microbialite mounds that reveal complex and irregular shapes and occur within three shallowing upward sequences capped by paleosols. These microbialite mounds are commonly constructed from smaller and more tabular-shaped mounds. Many of these small mounds developed around trees and branches which are preserved as moulds (or silicified wood) that are surrounded by a burrowed collar. Subsequently a thrombolite framework developed on the upper part only within bedded inter-mound packstones-grainstones. Horizontal stratification on-lapping and inter-fingering of the moderately porous inter-mound facies with the mounds suggest that deposition occurred contemporaneously with the development of the mound. The controls on these deposits were studied through Mounds3D, a SFM showing that mound distribution and their characteristics can be explained as a balance between the intrinsic properties of the microbial community and extrinsic environmental processes.

This study emphasises the importance of interrelated fieldwork-based studies and numerical modelling to produce robust depositional models. The predictive value of this study is enhanced by the multi-scale approach of both disciplines.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016