Geophysical characterisation and improved delineation of coaly source rocks from integrated analysis of laboratory, well and seismic data
Tusar R. Sahoo
A B * , Richard Sykes A , Ludmila Adam C and Robert H. Funnell A
A GNS Science, Lower Hutt, 5010, New Zealand.
B University of Canterbury, Christchurch, 8140, New Zealand.
C University of Auckland, Auckland, 1142, New Zealand.
The APPEA Journal 63 S269-S274 https://doi.org/10.1071/AJ22100
Accepted: 20 February 2023 Published: 11 May 2023
© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of APPEA.
Abstract
Coaly facies (coal, shaly coal and coaly mudstone) are proven petroleum source rocks in many sedimentary basins in New Zealand, Australia and Southeast Asia. Often these source rocks are mapped in the subsurface based on sparse well data and seismic amplitude character and their organic richness is estimated using average TOC (total organic content) values from well data. However, as the lateral distribution and organic richness of coaly facies are highly heterogeneous and the seismic amplitude response of facies is non-unique, delineation of coaly facies and TOC estimation away from wells are highly uncertain. To reduce this uncertainty, we characterised coaly facies in Cretaceous–Eocene intervals in Taranaki and Great South Basins using density, P-wave velocity, P-impedance and TOC data from 13 wells, supplemented with rock physics and TOC data of numerous coal samples collected from mines and outcrops around New Zealand. We then carried out a case study from the Paleocene–Eocene interval in the Maari 3D seismic survey area in offshore Taranaki Basin in which we develop a P-impedance model and delineate the coaly facies away from wells based on P-impedance character. Average P-impedance maps were analysed together with seismic amplitude character, structure maps and depositional environment maps to understand lateral and temporal variation of coaly facies. These maps were then converted to proxy-TOC maps using a relationship developed from the outcrop coal samples. The coaly facies show density <2.5 g/cc, P-wave velocity <4000 m/s, P-impedance <9000 m/s × g/cc and varied TOC character in the Cretaceous–Eocene interval.
Keywords: coal, coaly facies, delineation of coaly source rocks, Great South Basin, Maari 3D, seismic inversion, source rock, Taranaki Basin, TOC estimation.
Tusar Sahoo is a Geoscientist in the Earth Resources and Materials Department at GNS Science. After completing his MTech degree in Petroleum Exploration at the Indian School of Mines in 2006, he joined Reliance Industries Limited, as a Petroleum Geoscientist and was working on evaluation of hydrocarbon prospectivity of several basins in India. He joined GNS Science in 2012, and has been focussing his studies on structural evolution, seismic attribute analysis, play fairway analysis, paleogeographic evolution and petroleum prospectivity analysis of the offshore Canterbury, Great South and Taranaki basins in New Zealand. |
Richard Sykes is a Principal Scientist and Organic Geochemist at GNS Science, New Zealand. Over an almost 40 years career, his main research interests have focussed on understanding the origin and properties of coal deposits and petroleum fluids in New Zealand’s sedimentary basins. He also provides specialist geochemistry expertise to oil and gas exploration and production companies to help them better understand subsurface petroleum plumbing systems and ensure safe production. |
Ludmila Adam is a Senior Lecturer in the School of the Environment at the University of Auckland specialising in rock physics. She was graduated from Colorado School of Mines. Her research career started in petroleum reservoirs and has since expanded into geological sequestration of CO2, geothermal energy, geologic hazards and planetary geophysics. |
Rob H. Funnell is an Emeritus Scientist at GNS Science, New Zealand. He specialises in thermal modelling of sedimentary basins, the development of basin models to quantify petroleum prospectivity and risk, and storage aspects of carbon dioxide sequestration. Rob has undertaken a range of programme, objective, project and team leadership roles within GNS in the last 37 years. |
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