Abstract:
Following decades of technological innovation, geologists now have access to extensive 3D seismic surveys across sedimentary basins. Using these voluminous data sets to better understand subsurface complexity relies on developing seismic stratigraphic workflows that allow very high-resolution interpretation within a cost-effective timeframe. We have developed an innovative 3D seismic interpretation workflow that combines fullvolume and semi-automated horizon tracking with high-resolution 3D seismic stratigraphic analysis. The workflow consists of converting data from seismic (two-way traveltime) to a relative geological time (RGT) volume, in which a relative geological age is assigned to each point of the volume. The generation of a horizon stack is used to extract an unlimited number of chronostratigraphic surfaces (i.e., seismic horizons). Integrated stratigraphic tools may be used to navigate throughout the 3D seismic data to pick seismic unconformities using standard seismic stratigraphic principles in combination with geometric attributes. Here, we applied this workflow to a high-quality 3D seismic data set located in the Northern Carnarvon Basin (North West Shelf, Australia) and provided an example of high-resolution seismic stratigraphic interpretation from an Early Cretaceous shelfmargin system (Lower Barrow Group). This approach is used to identify 73 seismic sequences (i.e., clinothems) bounded by 74 seismic unconformities. Each clinothem presents an average duration of approximately 63,000 years (fifth stratigraphic order), which represents an unprecedented scale of observation for a Cretaceous depositional system on seismic data. This level of interpretation has a variety of applications, including high-resolution paleogeographical reconstructions and quantitative analysis of subsurface data. This innovative workflow constitutes a new step in seismic stratigraphy because it enables interpreters to map seismic sequences in a true 3D environment by taking into account the full variability of depositional systems at high frequency through time and space. Introduction Since the first breakthrough in seismic stratigraphic interpretation, seismic data have proved to be the most fundamental tool for basin analysis and petroleum exploration (Payton, 1977). The introduction of 3D seismic data in the late 1980s and the subsequent development of workstation-based processing and interpretation tools in the 1990s and 2000s led to a revolution in earth sciences, with industrial and academic applications (Nestvold, 1996; Weimer and Davis, 1996; Dorn, 1998; Davies et al., 2004; Chopra and Herron, 2010; Brown, 2011). Technological innovations during the past two decades have allowed geoscientists to acquire and interpret extensive high-quality 3D seismic surveys, hence improving our understanding of the stratigraphy and structural geology of the subsurface and providing unprecedented insights into the composition and evolution of sedimentary basins (Hart, 1999; Posamentier, 2000; Davies et al., 2004; Cartwright and Huuse, 2005). Indeed, one of the main problems able to be tackled through the introduction of 3D seismic data was the spatial resolution, which increased from kilometer scale (with 2D seismic data) to 25 m or less (with 3D seismic data), enabling geoscientists to visualize “small” elements of depositional systems (e.g., drainage networks; Posamentier, 2004). This finer scale imaging resolution combined with the possibility of processing complex seismic trace information using advanced algorithms (i.e., seismic attribute mapping; Chopra and Marfurt, 2007) led to the development of seismic The University of Western Australia, Centre for Energy Geoscience, School of Earth Sciences, 35 Stirling Highway, Perth, Western Australia 6009, Australia. E-mail: victorien.paumard@uwa.edu.au; julien.bourget@uwa.edu.au; annette.george@uwa.edu.au; simon.lang@uwa.edu.au. Eliis SAS, Parc Mermoz, Immeuble l’Onyx, 187, Rue Hélène Boucher, Castelnau-le-Lez 37170, France. E-mail: benjamin.durot@eliis.fr; sebastien.lacaze@eliis.fr. Chevron Australia Pty. Ltd., 250 St. Georges Terrace, Perth, Western Australia, 6000, Australia. E-mail: tobi.payenberg@chevron.com. Manuscript received by the Editor 12 October 2018; revised manuscript received 9 February 2019; published ahead of production 23 April 2019; published online 03 July 2019. This paper appears in Interpretation, Vol. 7, No. 3 (August 2019); p. B33–B47, 13 FIGS. http://dx.doi.org/10.1190/INT-2018-0184.1. © 2019 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved. t Tools, techniques, and tutorials Interpretation / August 2019 B33 geomorphology (Posamentier, 2000, 2004). Thus, seismic stratigraphy was not only limited anymore to 2D mapping of seismic discontinuities and seismic facies, but it would also include high-resolution mapping of the depositional geomorphology contained within each seismic sequence (Zeng, 2018). The main constraints associated with these extensive (>10;000 km2) and high-resolution data sets are to find the appropriate tools to interpret these data in a cost-effective timeframe (Cartwright and Huuse, 2005). For instance, until very recently, seismic interpreters applied traditional picking methods of key seismic horizons (e.g., interpretation every 10 inlines and crosslines), which represented a large amount of the time spent on the interpretation of a 3D seismic volume (Pauget et al., 2009). Reducing this “picking” work to spend more time on the geological analysis and understanding of the data is an important challenge, particularly in the oil and gas industry. The recent development of a new generation of full-volume, semi-automatic, seismic interpretation tools available in commercial software packages allows reducing the time spent on manual picking (De Groot et al., 2010; Hoyes and Cheret, 2011; Stark et al., 2013; Qayyum et al., 2018). These tools rely on advanced algorithmsbased methods to simultaneously autotrack a high number of seismic horizons throughout 3D volumes (Pauget et al., 2009; Fomel, 2010; Labrunye and Carn, 2015; Wu and Hale, 2015). These methods result in the creation of a relative geological time volume in which each point of the 3D seismic data is associated with a relative geological age (Stark, 2004). It means that the mapping of seismic sequences can be undertaken at very high resolution, in a real 3D interpretation framework, by using standard seismic stratigraphic principles and geometric attributes (Van Hoek et al., 2010). This constitutes a major advance in seismic interpretation because 3D full-volume mapping of seismic stratigraphic unconformities and sequences provides more accurate solutions than 2D manual picking (on inlines/crosslines) and because it enables interpreters to characterize lateral and vertical changes in sediment thicknesses and stratal stacking patterns at an unprecedented fine resolution (De Groot et al., 2010). This paper uses high-quality 3D seismic data located in the Northern Carnarvon Basin (North West Shelf, Australia) to interpret a high-resolution seismic stratigraphic framework of the Lower Barrow Group (LBG) (Figure 1). The LBG constitutes a Late Jurassic–Early Cretaceous shelf-slope-basin system (approximately 100–500 m high clinoforms) that was deposited during a late syn-rift tectonic phase (Figure 2; Paumard et al., 2018). At the basin scale, the stratigraphic evolution of the LBG comprises six third-order seismic sequences that present significant along-strike variability due to lateral variations in subsidence regime and shifts in sediment supply as a result of the active rift setting (Paumard et al., 2018). Using standard manual picking methods to interpret seismic horizons in the LBG falls short in two aspects. First, to map a very high number of shelf-margin sequences (fourth to fifth order) in a reasonable timeframe, the interpreter has to focus on a few selected inlines and crosslines, and/or work on a subsample of the 3D volume. Either way, this will result in a significant loss of geological information especially regarding alongstrike changes in stratal stacking pattern and geometry across single high-order seismic sequences. Second, a traditional high-resolution seismic stratigraphic interpretation of these data (i.e., based on a few dip-oriented seismic lines extracted from the volume) will be model-driven (e.g., based on the identification of seismic sequences attached to system tracts geometries) that will be tentatively correlated from one line to another. This can result in the erroneous correlation of seismic packages that are not genetically related, hence resulting in a seismic stratigraphic interpretation that does not take into account the full lateral variability of the strata (Madof et al., 2016). To overcome these issues, this paper presents an advanced workflow based on a full-volume, semi-automated seismic Figure 1. Location map of the study area. The background map, and the inset of Australia, corresponds to bathymetry (meter below modern sea level) and topography (meter above modern sea level) at 250 m resolution, obtained from the Geoscience Australia database. The map within the 2D seismic outline corresponds to the seafloor horizon interpreted and gridded on 2D seismic data. The white outlines highlight the geological provinces (i.e., Investigator Depocentre, Exmouth Terrace, Exmouth Depocentre, and Barrow Depocentre) recording deposition of the LBG during the latest Jurassic–Early Cretaceous. B34 Interpretation / August 2019 interpretation software. The approach is twofold: (1) conduct a full-volume seismic interpretation workflow within each one of the third-order seismic sequences of the LBG (Figure 2); and (2) from a highresolution relative geological time (RGT) model, use integrated stratigraphic tools to identify and map several significant chronostratigraphic surfaces (i.e., seismic unconfo
