The Diagenesis Of Permian Arenaceous And Argillaceous Units In The Moomba Gas Field, Southern Cooper Basin, South Australia
Degree of Master of Science, 1990
University of Adelaide
Core logging, thin section description, scanning electron microscopy, X-ray diffraction and cathodoluminescence have been used to understand the diagenetic changes which have occurred in the Permian sequence of the Moomba Gas Field. Core logging confirms previously published accounts of environments of deposition for the Patchawarra, Murteree, Daralingie and Toolachee Formations.
Euhedral prisms and drusey quartz overgrowths were initiated early but continued through several stages of the diagenetic history. Quartz overgrowths are common in clean medium to coarse-grained sands of all formations studied but absent in fine sands and silts where illite prevents their development. Early quartz overgrowths provided enough rigidity to prevent subsequent compaction and allowed retention of some primary porosity.
Clays of detrital or authigenic origin are detrimental to reservoir quality. Illite and kaolinite occurring in fine-grained sands and silts considerably reduce porosity and permeability.
Authigenic dickite, precipitated from solution, is the dominant kaolin mineral in coarser-grained sands. Microporosity associated with pore filling dickite accounts for a significant proportion of total porosity found in reservoir sands. Up to 504 of porosity is microporosity which is productive for gas.
Mg-rich siderite and Fe-rich dolomite are the carbonate minerals found, but these generally occur in only minor proportions. They exhibit a variety of crystal habits which reflect mode and timing of formation. Carbonate authigenesis reduces porosity and permeability although restricted dissolution may be of local significance in enhancing reservoir quality.
Dissolution of labile grains is widespread throughout the Permian sands studied and contributes to increased reservoir quality in point bar deposits in particular. Porosity and permeability are greater for the coarser sands which are frequently found in point bars.
Traces of bitumen discovered in many samples point to oil migration into the Moomba structure, although the total quantity may have been small. This oil was probably displaced from the Moomba structure by gas. Gas emplacement is considered to be the final diagenetic event which has occurred in the Field.
Clay minerals present in reservoir units control electric log response and affect porosity and water saturation determinations. Gamma Ray and Sonic log responses are affected by illite and kaolin respectively.
The presence of Fe and Ca carbonates is not thought to adversely affect any proposed acid stimulation programme. Hydraulic fracturing may be a preferred method of stimulation, particularly in tight sands where porosity may be wholly composed of microporosity associated with pore filling kaolin.