Delineation of coaly source rock distribution and prediction of organic richness from integrated analysis of seismic and well data

https://doi.org/10.1016/j.marpetgeo.2020.104873Get rights and content

Highlights

  • A new method was developed to better delineate coaly facies from 3D seismic data.

  • Relationships established between density, TOC and P-impedance of coaly rocks.

  • Average P-impedance maps transformed to average TOC maps for seismic intervals.

  • Improved quantification of coaly source rocks for petroleum systems modelling.

Abstract

Coaly facies (coals, shaly coals and coaly mudstones) are the primary source rock in several important petroleum basins in SE Asia and Australasia. Delineating these facies and quantifying their organic richness within kitchen areas are some of the key risks for petroleum exploration. To improve their delineation and characterisation using seismic data, a case study was undertaken of the coal-bearing Paleocene and Eocene intervals within the Maari 3D seismic volume area, offshore southern Taranaki Basin, New Zealand. Supporting data were acquired from well logs (including borehole image logs), existing geochemical analyses of well cuttings, and new laboratory measurements of in-situ geophysical and geochemical properties of coal samples from onshore coal mines and other outcrops. In the Paleocene–Eocene interval in the Maari 3D, coaly facies display characteristic low density (<2.5 g/cc) and velocity (<4000 m/s) and appear as bright, resistive layers in image logs. To delineate these facies in the Maari 3D volume, a post-stack seismic inversion was carried out and a P-impedance (product of density and P-wave velocity) model was prepared. Coaly facies exhibit moderate–high amplitude and moderately continuous (up to ~5 km) seismic reflectors, with low P-impedance (<9000 m/s × g/cc) character. Interval P-impedance and amplitude maps reveal temporal variation in the distribution and abundance of coaly facies within the Paleocene–Eocene coastal plain depositional environment. A logarithmic relationship between P-impedance and TOC was used to convert average P-impedance maps to proxy TOC maps for five Paleocene–Eocene time intervals. Coaly facies are most abundant in the low P-impedance areas in the southwestern and central parts of the study area where average TOC is estimated up to 30 wt.%. P-impedance-derived estimates of TOC may be less reliable in structurally high areas hosting hydrocarbon-bearing sandstones and siltstones as some of these facies show similar P-impedance character to coaly facies.

Introduction

Identification and delineation of petroleum source rocks is a key aspect of petroleum systems evaluation, yet most basin modelling studies lack reliable information on the distribution, volume and richness of the source rock. Typically, source rocks have been delineated by analysing well data, seismic attributes and depositional environments, and source rock richness (i.e., total organic carbon content) has been estimated by taking averages from sparse well calibrations (e.g., Peters et al., 2005; Badics et al., 2015; Kroeger et al., 2015). However, there remains large uncertainty in the delineation and characterisation of source rocks in the absence of well data calibration of seismic facies, especially since different facies can show similar amplitude responses. Simple averages of total organic carbon (TOC) at well locations within a stratigraphic interval typically do not provide sufficiently representative information on the distribution of source rock richness for the target petroleum system. Not only are wells rarely drilled through source rock intervals, but wells are generally widely spaced and cannot adequately capture the typically large lateral variation in source rock richness (e.g., Bohacs and Suter, 1997), especially in terrestrial, coaly source rock systems.

Several new seismic inversion data techniques have recently been developed for characterising source rocks (Løseth et al., 2011; Chopra et al., 2012; Ogiesoba and Hammes, 2014; Badics et al., 2015; Amato del Monte et al., 2018). Most of these were developed for shale gas exploration and conventional shale source rocks. In contrast, few studies have focused on identifying and delineating conventional coaly source rocks, and even fewer have attempted to use seismic inversion techniques to estimate the organic richness of coaly source rocks (Løseth et al., 2011) using acoustic properties such as P-impedance (i.e., the product of density and P-wave velocity).

Cretaceous–Cenozoic coaly source rocks have generated large volumes of oil and gas in many sedimentary basins, particularly in Australasia and southeast Asia (Field and Browne, 1989; Noble et al., 1991; Moore et al., 1992; Cook et al., 1999; Sykes et al., 2014a). Mapping and characterising coaly source rock facies are therefore key issues for ongoing exploration and exploitation in many terrestrial-sourced basins. This paper presents a study to better delineate coaly facies and estimate their organic richness within the Maari 3D seismic data volume, offshore southern Taranaki Basin, New Zealand (Fig. 1). The term coaly facies is used in this study to refer to the group of three broad coaly lithologies – coal, shaly coal and coaly mudstone – that collectively constitute the continuum of coaly source rocks (Sykes and Raine, 2008, Sykes and Snowdon, 2002). Well data show that the coaly facies display wide variability in their vertical and lateral distributions, locally and regionally. An integrated analysis of well logs, image logs, seismic data, well cuttings descriptions and geochemical data was carried out to map the distribution and quantify the organic richness of coaly facies within the coal-bearing Paleocene–Eocene interval imaged in the Maari 3D seismic volume. The first step was to determine the density and velocity cut-off values for the three coaly lithologies so that impedance could be used to define coaly facies in seismic inversion-derived impedance volumes. Seismic reflection character, depositional environment and P-impedance were then integrated to identify the spatial distribution of coaly facies within the Paleocene–Eocene coastal plain depositional setting. Lastly, the variability in organic richness within this setting was mapped by developing a proxy TOC volume from inverted P-impedance data for the Maari 3D area.

It is important to note that Paleocene–Eocene coal measures are not the source interval for the black oil reserves within Maari Field. Biomarker studies indicate that the oil is derived primarily from Late Cretaceous coaly rocks of the Rakopi and possibly North Cape formations (Killops et al., 1994; Sykes et al., 2012). Rather, the Paleocene–Eocene interval in the Maari 3D volume was selected for study because it provides recent, high-quality seismic and well data with which to develop and test our methodology. This methodology could be equally applied to the Cretaceous and other coaly source rock intervals in other areas of Taranaki and other basins with suitable well and seismic data.

Section snippets

Geological setting

The Taranaki Basin is primarily located offshore west of the North Island of New Zealand (Fig. 1) and for more than five decades has been the main focus area for oil and gas exploration and production in New Zealand. The evolution of this basin has been well studied (Knox, 1982; Holt and Stern, 1994; King and Thrasher, 1996; Giba et al., 2010; Reilly et al., 2015; Strogen et al., 2017). The basin developed during the Cretaceous–Cenozoic and has an early rift-drift history associated with the

Data

Active petroleum exploration in the Taranaki Basin over the last five decades has generated a large amount of openfile 2D and 3D seismic reflection data, well data and associated reports that are available from New Zealand Petroleum and Minerals (https://data.nzpam.govt.nz). In this study, we used data from nine wells (Matuku-1, Pukeko-1, Te Whatu-2, Kea-1, Maari-1, Moki-1, Manaia-2/2A, Whio-1 and Maui-4), the Maari 3D seismic data (PGS Data Processing, 2009), eight horizon grids (Thrasher et

Delineation and characterisation of coaly facies from well log data

Density, velocity, resistivity and gamma ray values characteristic of different coaly lithologies are summarised in Table 1. In general, they are characterised by low density, low velocity and high resistivity (>7 ohmm), whereas gamma ray does not appear to be particularly discriminatory for these facies. The density cut-off values of coal (<1.5 g/cc) and shaly coal (1.5–2 g/cc) are adopted from Sykes and Snowdon (2002) and Sykes (2014). Cross-plot analysis of well logs in the Paleocene and

Depositional controls on distribution of coaly facies

Accumulation of significant volumes of coaly facies in a sedimentary succession primarily depends on (1) growth of vegetation, (2) preservation of organic matter, (3) restricted dilution by clastic sediments, and (4) creation of accommodation space (Bohacs and Suter, 1997; Peters et al., 2000; Bohacs et al., 2005). The depositional environment of the Paleocene–Eocene section in the Maari 3D area was a predominantly terrestrial coastal plain setting (Fig. 12e) (King and Thrasher, 1996; Strogen,

Conclusions

Integrated analysis of well log, geochemical and seismic data has enabled the delineation of coaly facies and estimation of average organic richness (TOC) within the Paleocene–Eocene interval of the Maari 3D seismic volume. We think our approach can help predict source rock distribution where good quality seismic data is available and is supported by geochemistry, conventional log and image log analysis. Coal and shaly coal lithologies are best defined at well sites using a combination of

Credit author statement

Tusar R. Sahoo: Conceptualization, Data curation, Formal analysis, Methodology, Visualization, writing- original draft and subsequent editing, Robert H. Funnell: Supervision, Writing – review & editing, Stephen W. Brennan: collection and analysis of coal samples, Richard Sykes: Funding acquisition, analysis of coal samples, Writing – review & editing, Glenn P. Thrasher: Supervision, Ludmila Adam: review, Mark J.F. Lawrence: image log analysis and review, Richard L. Kellett: review, Xiajing Ma:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was primarily funded by the Ministry of Business, Innovation and Employment (MBIE), New Zealand, as part of the GNS Science-led programme “Understanding petroleum source rocks, fluids, and plumbing systems in New Zealand basins: a critical basis for future oil and gas discoveries” (Contract C05X1507). We would like to thank several mine operators which provided access to the outcrop samples. CRL Energy (now Verum Group; Lower Hutt, New Zealand) undertook density analyses of outcrop

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