a b s t r a c tThe chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end-Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic.Conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows. Changhsingian-Griesbachian: first Hindeodus parvus, and first appearance of foraminifers Postcladella kalhori and Earlandia sp. Griesbachian-Dienerian: first Neospathodus dieneri, and last appearance of foraminifer P. grandis. Dienerian-Smithian: first Novispathodus waageni and late Dienerian first appearance of foraminifer Hoyenella ex gr. sinensis. Smithian-Spathian: first Nv? crassatus and last appearance of foraminifers Arenovidalina n. sp. and Glomospirella cf. vulgaris. Spathian-Aegean: first Chiosella timorensis and first appearance of foraminifer Meandrospira dinarica. Aegean-Bithynian: first Nicoraella germanica and first appearance of foraminifer Pilammina densa. Bithynian-Pelsonian: after last Neogondolella regalis, prior to first Paragondolella bulgarica and first appearance of foraminifer Aulotortus eotriasicus. Pelsonian-Illyrian: first Pg. excelsa and last appearance of foraminifers Meandrospira? deformata and Pilamminella grandis. Illyrian-Fassanian: first Budurovignathus truempyi, and first appearance of foraminifers Abriolina mediterranea and Paleolituonella meridionalis. FassanianLongobardian: first Bv. mungoensis and last appearance of foraminifer A. mediterranea. Longobardian-C ordevolian: first Quadralella polygnathiformis and last appearance of foraminifers Turriglomina mesotriasica and Endotriadella wirzi.The section contains primary magnetic signature with frequent reversals occurring around the Permian-Triassic, Olenekian-Anisian, and Anisian-Ladinian boundaries. Predominantly normal polarity occurs in the lower Smithian, Bithynian, and Longobardian-Cordevolian. Predominantly reversed polarity occurs in the upper Griesbachian, Induan-Olenekian, Pelsonian and lower Illyrian. Reversals match well with the GPTS. Large amplitude carbon isotope excursions, attaining values as low as À2.9‰ d 13 C and high as +5.7‰ d 13 C, characterize the Lower Triassic and basal Anisian. Values stabilize around +2‰ d 13 C through the Anisian to Carnian. Similar signatures have been reported globally. Magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the Lower Triassic and an overall decline in magnitude of fluctuation through the Middle and Upper Triassic. The largest spikes in magnetic susceptibility and gamma ray, indicating greater terrestrial lithogenic flux, correspond to positive d 13 C excursions. High precision U-Pb analysis of zircons from volcanic ash beds provide a robust age of 247.28 ± 0.12 Ma for the Olenekian-Anisian boundary at Guandao and an age of 251.985 ± 0.097 Ma for the Permian-Triassic boundary at Taiping. Together, the new U-Pb geochronology from the Guandao and Taiping sectio...
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.
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