Application of chemostratigraphy to the Mungaroo Formation, the Gorgon field, offshore northwest Australia

Abstract

The Mungaroo Formation in the Gorgon Field is a stratigraphically complex fluvial system of Triassic age. It is also a major hydrocarbon reservoir, therefore understanding its internal stratigraphic architecture is of paramount importance to exploitation of its reserves. Here, the technique of chemostratigraphy is used to construct a correlation framework for the Mungaroo Formation of the Gorgon Field. Chemostratigraphy is a tool that employs variations in inorganic whole rock geochemistry to enable the characterisation and subsequent correlation of sediments. For this study, a total of 1,514 cuttings and core samples from eight wells in the Gorgon Field have been analysed. Using data derived from both claystone and sandstone lithologies, the Mungaroo Formation is divided into nine chemostratigraphic packages, 22 geochemical units and 19 sand units. Additionally, three surfaces identified as time lines (T1–T3) are geochemically defined. Changes in values of Ga/Rb and Al2O3/(CaO+ MgO+K2O+Na2O) indicate that during deposition of the Mungaroo Formation, the paleoclimate became warmer and wetter, resulting in increasingly intense hydrolytic weathering. Steps in the values of these ratios allow three surfaces to be identified (T1–T3), at which there is a marked and sustained change in the paleoclimate. These three surfaces represent time lines that provide a quasi-chronostratigraphic framework for the formation. Values of Cr/Al2O3, Cr/Na2O and Nb/Al2O3 are related to changes in sediment provenance and indicate that during deposition of the Mungaroo Formation the provenance became more mafic and less intermediate. It is variations in paleoclimate and provenance modelled from the geochemical data that allows the packages, units and sand units to be characterised and correlated. The chemostratigraphic correlation is more detailed than is available from other stratigraphic techniques. Although in most instances the lithostratigraphic correlation of sand units based on wireline log correlation matches the one defined using chemostratigraphy, there are some significant differences between the two that influence reservoir models and gas production.

Sedimentology, geochemistry and paleoenvironmental reconstruction of the Cretaceous Yolde formation from Yola Sub-basin, Northern Benue Trough, NE Nigeria

“…is associated with illite or glauconite (Ratcliffe et al, 2010). Titanium (Ti) has known association with terrestrial detritus in high energy environments (Ross and Bustin, 2009) and mostly originates from mixed clay assemblage.…”

Using Elemental Chemostratigraphy on Mid–Late Frasnian Platform-Top Successions from the Lennard Shelf Outcrops, Canning Basin, Western Australia

“…Published siliciclastic chemostratigraphic papers define chemostratigraphic packages (e.g., Ratcliffe et al 2010Ratcliffe et al , 2015 or chemozones (e.g., Ramkumar et al 2011, Craigie 2015 as the primary correlative features. Both chemostratigraphic packages and chemozones are defined as having distinctive element or element ratio values when compared to the rest of a section.…”

“…However, the MgO values, as discussed already, are controlled by the amount of dolomitization, a feature that is unlikely to be of regional stratigraphic significance because dolomite bodies are not correlative over short distances in the field. Therefore, as discussed by many chemostratigraphic workers (e.g., Ratcliffe et al 2010Ratcliffe et al , 2015, if package definition is to be stratigraphically useful, it needs to be based on element ratios that are related to changes in parameters that are linked to stratigraphy. Figure 7, where it is apparent that the amount of variation between closely spaced samples is typically large when compared to the overall variation in the entire section, resulting in chemical logs that lack systematic variation.…”

Ungani Oil Field, Canning Basin - Evaluation of a Dolomite Reservoir

“…Well chemostratigraphy correlation (Ratcliffe et al 2010) was performed using ditch cuttings and core chips samples using high precision elemental data collected for 50 elements by Inductively Coupled Plasma -Optical Emission Spectroscopy -Mass Spectrometry (ICP-OES-MS, Jarvis and Jarvis 1992a & b). Elemental data was also used for a chemostratigraphic correlation giving information on bulk lithology and a fingerprint of clastic provenance (e.g.…”