The Jurassic and Early Cretaceous Magmatic Record of the Northern Carnarvon Basin, Western Australia: Implications for Petroleum Exploration and Development

Abstract

The Northern Carnarvon Basin (NCB) is formed during the rifting and split of Greater India from Australia during the breakup of Gondwana between the Late Jurassic and Early Cretaceous. During this period, a significant volume of magma was emplaced into the crust of the NCB. Since the first (and only) regional studies of the NCB’s syn-rift, pre-breakup, magmatic system (NCBMS) in the late 1990s, our knowledge of the processes of magma emplacement into sedimentary basins has evolved considerably. Furthermore, in this time, the region host to the NCBMS has been covered by almost continuous closely spaced 2D and 3D seismic surveys. These developments have enabled the research that is presented here: The first detailed assessment of the temporal and spatial distribution of the NCBMS at the basin scale in this modern context, and assessment of its impact on petroleum exploration and development. The intrusive component of the NCBMS is here mapped in unprecedented detail. Igneous intrusions, emplaced between the Kimmeridgian and the Valanginian, occur over ~40,000 km2 of the Exmouth Plateau and Exmouth Sub-Basin. The morphology of intrusions is likely governed by host rock mechanical properties. Fluidisation of compliant coals and mudstones, which are interbedded with competent sandstones in the Triassic Mungaroo Formation, enabled growth of the network of colossal (170+ km long) stacked, sheet-like sills mapped in the western Exmouth Plateau and southern Exmouth Sub-Basin. In contrast, formation of smaller (4 – 60 km in diameter) saucer shaped sill intrusions was promoted in homogenous Jurassic marine shales deposited in the central and northern Exmouth Sub-Basin. The preservation histories of the NCBMS’ two remaining volcanic centres: the Toro Volcanic Complex (TVC), and the Pyrenees Volcano, are here told in detail. The current setting of these Tithonian-aged volcanic centres suggests that they were subject to significant uplift and subsequent erosion during the formation of the Ningaloo and Novara arches in the Early Cretaceous. The increasing volume of material removed from the TVC towards the south, across the Ningaloo Arch beneath a regional breakup unconformity, suggests that the TVC was once much larger, extending into the southern Exmouth Sub-Basin. It is striking that the NCB’s preserved intrusive:extrusive rock ratio is minute compared to other volcanic rifted margins, where between 1/4 and 1/3 of magma is often erupted; if the TVC was originally significantly larger, this may go toward rebalancing the ratio. In all cases in which the NCBMS is sampled by drilling, the igneous rocks are found to be hydrothermally altered to clay minerals. This has impacted petroleum exploration efforts in the NCB: Altered igneous rocks in the NCB are difficult to identify on seismic reflection data (leading to misidentification of overpressured formation), and in drill cuttings (leading to possible loss of information for thermal history modelling). In other cases, contact heating from a stack of 20+ intrusions has overmatured organic matter in potential source rocks, and the presence of volcanic ash altered to smectite, a swelling clay, has forced companies to incorporate the use of inhibited muds into their well designs.