Late Jurassic lacustrine carbonates; a multi-scale analysis of the Mupe Member (Purbeck Limestone Group) of the Wessex Basin, U.K. / Gallois, Arnaud.

2016. 496 p.

Research output: ThesisDoctoral Thesis

Unpublished

Documents

Abstract

The discovery of hydrocarbons in Lower Cretaceous, non-marine carbonates in the South Atlantic has triggered much research into understanding how such complex strata form. Sedimentary facies and basin architecture are controlled by a range of environmental parameters (i.e. climate, hydrology and tectonic setting) but published facies models are few and limited in their predictive value. This study develops new depositional models for non-marine microbialites and associated facies in a semi-arid climate setting in an extensional basin based on the Purbeck Limestone Group (Upper Jurassic – Lower Cretaceous) exposed in Dorset.
Outcrop studies coupled with subsurface, petrographic and petrophysical studies constrain and improve published facies models and palaeogeographical maps of the microbialites deposited in the syn-rift phase of the Wessex Basin. Two and three-dimensional seismic data imaging the Lower-Upper Jurassic in south Dorset show east-west trending south-dipping syndepositional extensional faults. These are linked via a relay ramp and new tectono-sedimentary models indicate tectonic controls on facies distribution during propagation of extensional faults. Accumulation of strata occurred in half-grabens controlled by these east-west extensional faults indicating that the facies within the Purbeck Limestone Group are, in part, controlled by this tectonic template. Outcrop study and remote sensing imaging (lidar) coupled with petrographic study is used to differentiate microbial mounds and bedded inter-mound packstone-grainstone areas and to characterise a total of 9 facies.
The limestones of the Mupe Member of the Purbeck Limestone Group represent four meter scale lacustrine cycles, deepening upwards and then shallowing upwards after a flooding surface, capped by emergent surfaces (paleosols). These sequences are characterised by accumulation of in-situ build-ups of highly porous microbial mounds that occur within less-porous bedded inter-mound packstones-grainstones. The microbial mounds are located in three stratigraphic units (“Skull”, “Hard” and “Soft Caps”) separated by three paleosols (“Basal”, “Lower” and “Great Dirt Beds”). These microbial mounds vary in thickness from about 0.2 to 3 m and preserve a high primary framework porosity. Many of the mounds developed around trees, tree trunks and branches which are preserved as moulds, or silicified wood and they comprise three sub-facies (Stromatolite, Thrombolite and Burrowed peloidal packstone). Horizontal stratification, onlapping and interdigitating of the inter-mound facies with the mounds suggest that deposition occurred contemporaneously with the development of the mounds and that they were low relief structures on the lake floor. Interpretation of high resolution ground-based lidar data collected from seven quarries in the Isle of Portland, together with some 3-D outcrops enables a quantitative description of the morphology of the mounds and their relationship with the packstone-grainstone inter-mound facies. Most of the facies are weakly porous at micro and macroscales apart from microbial mounds. These exhibit a rather high micro and macro-porosity and the best potential reservoir unit, however as they are disconnected spatially and bounded by paleosols, the quality of the overall potential reservoir remains poor.
The main goals of this project are to predict the controls on the location, shape and the size of microbial mounds and to constrain the facies models and palaeogeographies of the Mupe Member within the Wessex Basin. The main conclusions of this multi-scale study are (1) that the two main extensional faults in this part of the Wessex Basin were active during Purbeck time; (2) that the development of microbial mounds is tectonically controlled as indicated by their relationship with the relay ramp; (3) mound occurrence is controlled by palaeotopography generated on sub-aerial exposure surfaces, fossil soils and early conifer trees; (4) mounds are developed mainly on the shallowest area of the lake as indicated by their rounded to tabular shapes, and their relationship with the packstone-grainstone inter-mound facies and the paleosols; (5) petrographic, palaeontological and isotopic data indicate an open, through-flowing brackish water lacustrine system in a semi-arid, Mediterranean-type climatic setting; and the characterisation of potential reservoir shows (6) that reservoir qualities are overall relatively poor.
Original languageEnglish
QualificationPh.D.
Awarding Institution
Supervisors/Advisors
Thesis sponsors
  • BP Exploration Operating Company Ltd
  • Baker Hughes
Award date1 Mar 2017
Publication statusUnpublished - 2016
This open access research output is licenced under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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