Secondary Migration Fairways And Hydrocarbon Potential Of The Southern Enderby Terrace, Northern Carnarvon Basin, Western Australia.
Draper, Paul Christopher
Honours Degree, 1995
University of Adelaide
The Southern Enderby Terrace is an attractive exploration region due to a combination of relatively shallow water depth, reasonably close proximity to the mainland, and shallow depth to target reservoir units. However, the failure of Arabella #1 (1983) to recognise mature source rocks or indicate any signi6cant hydrocarbon shows from good quality reservoir units at a local high has severely downgraded the prospectivity of the terrace. This study largely agrees with the pessimistic hydrocarbon prospectivity assessment for the bulk of the Southern Enderby Terrace, but recognises a small area of highly prospective oil potential in vacant acreage.
Re-appraisal of the Southern Enderby Terrace, and the northern Candace Terrace, was performed in the light of an economically significant oil discovery at Stag #1 (1993) and, at the time of project inception, pending gazettal of vacant acreage (in November 1994) over much of the Southern Enderby Terrace. Secondary migration pathways from the sub-basin depocentres form the main risk to a play on the terraces. Therefore, analysis of the potential for oil migration onto the terraces greatly assisted an assessment of the study area's prospectivity for Jurassic oil. The study area extended beyond the southern and western boundary of the Southern Enderby Terrace, enabling examination of the complex terrace-bounding Flinders and Sholl Island fault systems, and the lateral coverage and juxtaposition of post-Triassic stratigraphy across these faults. This migration fairways analysis was achieved by interpretation of 1800 km of seismic on a relatively sparse grid, and was performed in conjunction with a hydrocarbon habitat study, utilising published literature and open-file data from 13 key wells.
Several high quality reservoir units were identified, but the requirements of an adequate seal, access to oil charge, and reasonable potential for a hydrocarbon trap within the study area narrowed the field to a primary reservoir target - the M. australis Sandstone. Its coverage is restricted to a small region in the north of the study area, with pinchout approximately one third of the way between Stag #1 and Arabella #1. The Muderong Shale provides the seal, and the most likely traps are broad, low-relief, 4-way dip closures, or 3-way dip closure against a bounding fault - possibly the Mermaid Fault. Migration onto the terraces from the Barrow Sub-basin is considered medium to high risk, but an up-dip-to-southeast trend to reservoir units on the terrace, and lack of hydrocarbons at Arabella #l, reduces the exploration prospectivity for most of the Southern Enderby Terrace to very high risk. Migration up-dip from the Dampier Sub-basin margin into the M. australis Sandstone is the most likely scenario, giving this small region south of Stag #1 a low to medium exploration risk. The oil charge for the Stag discovery, and hence the study area, most likely occurred during the Turonian, and/or the Mid Miocene, being sourced from the Upper Jurassic Dingo Claystone of the southern Lewis Trough.
Detailed appraisal of gazettal permit areas suggests highest priority being placed on blocks 315 and 316 of permit W94-11, and the north-east corner block of permit T94-2.