Fluid flow through carbonate platforms as evidence for deep-seated reservoirs in Northwest Australia

Abstract

Fluid flow features in carbonate platforms of the outer Browse Basin, Northwest Australia, are investigated using 3D seismic and borehole data. During the Cenozoic, the study area evolved from a carbonate ramp to a rimmed platform with isolated carbonate build-ups; as such it played a key role in focusing fluid on their buoyant flow to the surface. Statistical analyses of direct hydrocarbon indicators show fluid flow to be focused in elevated areas i.e., the greater is the focusing of fluid, the larger is the carbonate platform or isolated build-up. Locally, karst systems created regions of enhanced permeability through the evolving carbonate stratigraphy. Karstified horizons are located on the top of carbonate build-ups and platform clinoforms as fluid buoyantly concentrates and diffuses up into topographic highs. In turn, the putative migration of gas and fluid generated hypogenic karst systems, enhancing permeability in otherwise lower permeability rock. Based on the interpreted data, we suggest a pivotal relationship between the presence of carbonate build-ups (and karsts) in the Browse Basin and hydrocarbon accumulations at depth. Areas of elevated topography focus sub-surface fluid, enhancing the permeability of carbonate successions. In turn, focused fluid flow can lead to the generation of methanogenic carbonates, enhancing the growth of isolated build-ups and leading to further generation of elevated features. This grants the identification of similar features on seismic data from Northwest Australia, and other Equatorial margins in the world, as a valid proxy for the recognition of hydrocarbon accumulations below thick carbonate successions.

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

“…Thereafter, a relative sealevel fall resulted in the accumulation of shallowmarine carbon ates and the evolution of a late Oligocene-Mio cene rimmed platform (Fig. 2;Apthorpe, 1988;Howarth and Alves, 2016;Belde et al, 2017). In summary, the Miocene and Pliocene sections of the Browse Basin represent an aggradational car bonate shelf.…”

“…The reactivation of Jurassic faults and re gional tectonic inversion that occurred after the onset of subduction in the Timor Trough resulted in highly variable deformation styles across the Browse Basin (Harrowfield and Keep, 2005). Smallscale normal faults crosscut late Oligo cene-Miocene strata in the eastern part of the study area, forming easttrending lineaments on the sea floor (Howarth and Alves, 2016). While no other faults offset the Miocene sequence, it is noted that subseismicscale fractures and faults may be present and control fluid flow.…”

“…Jurassic continental rifting between Greater India and Western Australia (Veevers and Cot teril, 1978;Langhi and Borel, 2008;Rosleff Soerensen et al, 2012) generated north easttrending structures on the North West Shelf, including in the Yampi, Leveque, Caswell, and Barcoo subbasins ( Fig. 1; Longley et al, 2002;Langhi and Borel, 2008;RosleffSoerensen et al, 2012RosleffSoerensen et al, , 2016Howarth and Alves, 2016). Early Cretaceous subsidence and the deposition of a passivemargin sequence buried the riftrelated topography (Fig.…”

“…An Eocene-early Oligocene carbonate ramp developed before a stratigraphic break recording uplift and erosion of the inner shelf (Fig. 2;Stephenson and Cadman, 1994;Howarth and Alves, 2016;Belde et al, 2017). Thereafter, a relative sealevel fall resulted in the accumulation of shallowmarine carbon ates and the evolution of a late Oligocene-Mio cene rimmed platform (Fig.…”

“…Locally, Howarth and Alves (2016) mapped clustered fluidflow features above late Oligo cene-Miocene karst systems and within iso lated carbonate buildups. These fluidflow fea tures were interpreted as being associated with a salt diapir at depth, despite: (1) the absence of thick evaporites in great parts of the Browse Basin, and (2) the presence of the shallowwater Seringapatam Reef in the area interpreted by Howarth and Alves (2016). While the latter au thors discussed fluid flow within (and above) the larger isolated carbonate buildups, they did not address the potential significance of migrating fluids on isolated carbonate buildup initiation and growth.…”