Sedimentological Control of Diagenetic Processes of the Tidal Sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia): Impact on the Reservoir Properties of the Petrel Gas Field

Abstract

Abstract

The deep silicoclastic reservoir (> 3500 m) of the Upper Cape Hay Formation of the Bonaparte Basin (Petrel gas field, Petrel sub-basin) exhibits wide heterogeneity in porosity (2 to 26%) and permeability (0.001 to 2500 mD). Factors controlling reservoir quality and heterogeneity – porosity (phi) and permeability (k) – are prerequisites for improving modelling and forecasting the gas reserves. To investigate this variability, 42 samples were taken from five wells drilled through the Upper part of the Cape Hay Formation. Six facies were identified from core description and microscopic study of the sandstones. These facies are typical of a large embayment estuary with tidal influence, and include (1) mud flat, (2) sand flat, (3) top of tidal sand bar, (4) middle of tidal sand bar, (5) base of tidal sand bar, and (6) upper offshore facies. The paragenetic diagenetic sequence comprises emplacement of precursor of ferrous chlorite coatings, mechanical compaction, chemical compaction, development of quartz overgrowth, feldspar alteration, quartz corrosion, formation of authigenic clay (kaolinite and illite), and ferrous calcite cementation. Mid and top of the tidal bars generally exhibiting the best porosity (phi > 10%) and permeability values (k > 0,1 mD). Diagenetically, the feldspar alteration and dissolution phase, promoted by acidic fluid circulation, occurred uniformly throughout the reservoir, representing 5 to 10% of the porosity of samples. Feldspar dissolution released silica and aluminium into the reservoir that promoted the development of kaolinite and illite, which tend to destroy porosity and permeability as calcite cement and quartz overgrowth. Chlorite coatings around detrital grains were systematically deposited within the sand bar facies at the end of the last third-order transgressive hemicycle of the Cape Hay Formation. In an estuarine environment, seawater and fresh water mixed in a mid-energy environment, thus promoting the development and conservation of a chlorite precursor around grains. This clay coating inhibited silicification and this proves to be the key parameter behind good phi-k qualities.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016