Carbonate platforms on equatorial margins: Geometry, evolution and importance on the world's equatorial margins

This thesis uses the high-quality Poseidon 3D seismic volume, 2D seismic data and well data from the Browse Basin, offshore Northwest Australia. It investigates the initiation, growth and demise of the Miocene Oliver Limestone Formation to understand the relative importance of different exogenic and endogenic controls on carbonate growth patterns. This thesis then goes on to assess the impact of scale relationships, associated with different datasets, on the characterisation of carbonate reservoirs. This is achieved through qualitative and quantitative interpretation of seismic data, outcrop and numerical models. The initiation of a carbonate system within the Browse Basin was first investigated to propose a new evolutionary model for Northwest Australia and to assess the relative significance of fluid flow and fault topography to carbonate build-up development. Faults and fluid flow features were analysed using detailed qualitative and quantitative seismic interpretation. The results demonstrate a positive correlation between fluid flow and the generation of pinnacle features on the Miocene sea floor of the Browse Basin. These pinnacle features are explained in this thesis as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates. They are considered as key features precluding the growth of the larger isolated carbonate buildups. This thesis postulates that pinnacle features above fluid-flow conduits demonstrate a valid seep-reef relationship and are refined diagnostic features for understanding fluid flow through geological time. The geometric and depositional responses of isolated carbonate build-ups to Miocene sea-level change and regional tectonics, was investigated to derive their relative significance on growth patterns and carbonate facies distribution. Seismic stratigraphy and facies associations were interpreted using 3D seismic and borehole data from the Browse Basin, Northwest Australia, combined with outcrop information from the Cariatiz Reef in Southeast Spain. Results documented Messinian structural partitioning to occur across the Browse Basin, with deformation associated with plate collision focused on preferentially oriented faults, i.e. only influencing carbonate buildup evolution at a local scale. Seismic attribute analyses proved highly effective in interpreting carbonate facies, but when compared with outcrop information from Southeast Spain, this same attribute data was limited to features larger than 17 m vertically and 50 m horizontally. Thus, estimations of reservoir potential are significantly underestimated when only based on the interpretation of seismic data. The final part of this thesis investigates the demise of carbonate build-ups in the Browse Basin so as to determine the relative significance of endogenic and exogenic processes in drowning carbonate factories. Growth patterns and drift deposits where interpreted using the high-resolution Poseidon 3D seismic survey and additional 2D seismic lines. The relative significance of different geological processes was investigated using numerical stratigraphic forward modelling (CarboCAT) to test different scenarios based on seismic interpretation and current literature. Results showed that the subsidence rates currently proposed in the literature generated accommodation space in the Browse Basin, but were of insufficient magnitude to drown the carbonate factories. Eustatic sea level did not influence large-scale growth patterns or drowned the build-ups. Hence, this thesis postulates that current activity (with associated deposition of sediment drifts) and excess nutrient supply were key to the drowning of carbonate build-ups in the Browse Basin. These two processes are often overlooked when interpreting the demise of carbonate build-ups on equatorial margins around the world. The overall implications of this work are as follows: 1) Fluid flow should be considered as an important endogenic control on carbonate build-up initiation and therefore the presence of carbonate build-ups in areas of hydrocarbon generation may act as an indicator of seal leakage; 2) Structural partitioning within the Browse Basin had a significant impact on carbonate growth patterns. Thus it is important to consider that localised tectonics can induce significant variability in growth patterns and, therefore, reservoir distribution within a basin; 3) Seismic attributes are a key tool in aiding the delineation of reservoir facies and processes generating secondary porosity (e.g. karsts). As such, they can aid the generation of increasingly robust geological models and thus reduce exploration uncertainty; 4) The primary cause of carbonate build-up demise during the Late Miocene is environmental deterioration as opposed to tectonic or eustatic sea level; 5) Numerical modelling (CarboCAT) proved an effective tool in testing qualitative interpretations derived from seismic interpretation. The numerical modelling completed in this work showed value in predicting reservoir distribution and carbonate growth patterns, parameters that can guide exploration on a basin scale, as well as aid interpretation at an inter-well scale to reduce exploration risk