Geometric and depositional responses of carbonate build-ups to Miocene sea level and regional tectonics offshore northwest Australia

The geometric and depositional responses of isolated carbonate build-ups to Miocene sea-level change and regional tectonics was investigated using a combination of 3D seismic and borehole data from the Browse Basin, North West Australia, and outcrop information from the Cariatiz Reef, southeast Spain. The interpreted seismic volume documents five (5) Miocene sequence boundaries and five (5) main seismic facies. Seismic attribute analyses proved a highly effective tool for interpreting carbonate facies but, when compared with outcrop information from southeast Spain, data are limited to large-scale features of scales beyond 16.4 m vertically and 18.75 m horizontally. Hence, this work clearly shows that estimations of reservoir potential are significantly underestimated if based on seismic data alone. As a corollary of the structural analysis in this work, growth patterns suggest Messinian structural partitioning across the Browse Basin, with deformation associated with plate collision focused in preferentially orientated faults - thus only influencing carbonate build-up evolution at a local scale

Pinnacle features at the base of isolated carbonate buildups marking point sources of fluid offshore Northwest Australia

We investigated pinnacle features at the base of late Oligocene–Miocene isolated carbonate buildups using three-dimensional seismic and borehole data from the Browse Basin, Northwest Australia. Brightened seismic reflections, dim spots, and other evidence of fluid accumulation occur below most pinnacle features. An important observation is that all pinnacles generated topography on successive late Oligocene–Miocene paleoseafloors, therefore forming preferential zones for the settlement of reef-building organisms by raising the paleo-seafloor into the photic zone. Their height ranges from 31 m to 174 m, for a volume varying from 33 km3 to 11,105 km3. Most of the pinnacles, however, are less than 2000 km3 in volume and present heights of 61–80 m. As a result of this work, pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates, and they are considered as precluding the growth of the larger isolated carbonate buildups. We postulate that pinnacle features above fluidflow conduits demonstrate a valid seep-reef relationship, and we propose them to be refined diagnostic features for understanding fluid flow through geological time

Strain decoupling reveals variable seismogenic risk in SE Japan (Nankai Trough)

The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional-compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike-slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved

Geomorphological evidence of carbonate build-up demise on equatorial margins: A case study from offshore northwest Australia

The demise of Miocene carbonate build-ups in the Browse Basin, Northwest Australia, has been explained as relating to geological and oceanographic processes. These include accelerated tectonic subsidence driven by subduction, ocean cooling following the mid-Miocene climate optimum, nutrient excess, poisoning by sediment drifts and local erosion driven by current winnowing, occurring discretely or simultaneously. Here, we critically assess the evidence for these different mechanisms using a combination of high-resolution 3-D seismic data, regional 2-D seismic profiles, and numerical stratigraphic forward modelling. Seismic interpretation and numerical modelling found that the proposed uniform subsidence rate of 125 m/Ma between 16.5 Ma and 5.33 Ma for the northern Browse Basin (Belde et al., 2017), when combined with the published estimate of eustatic sea level in Miller et al. (2005), was insufficient to drown the Miocene carbonate sequence and generate the geomorphological changes (barrier reef to isolated carbonate build-ups) observed on seismic data. Instead, a subsidence profile comprising pulses of rapid and slow subsidence is required. Significantly, our results suggest that subsidence rates exceeded 400 m/Ma in the northern Browse Basin, and that parts of the basin record the accumulation of sediment drifts. These sediment drifts are interpreted to have buried some carbonate build-ups, while suspended sediment reduced light transmissivity, inhibiting carbonate production. Thus, we postulate that current activity and excess nutrient supply are key, but often overlooked, oceanographic processes that lead to the demise of carbonate build-ups

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