Sedimentology, sequence stratigraphy and reservoir quality of the Early Cretaceous Murta Formation, Eromanga Basin, Central Australia.

Abstract

The Eromanga Basin hosts the Early Cretaceous reservoir sediments of the Murta Formation and its basal McKinlay Member of prograding fluvio-lacustrine and deltaic origin that are characterized by low oil production and recovery factors which are heavily controlled by depositional facies. The integration of the concepts of facies associations, sequence stratigraphy and petrography enabled this study to map the continuity of the Murta Formation and to point out the effect of the diagenetic features on the reservoir quality. The diagenetic effects and spatial distribution of the depositional facies in the basin are essential in nominating locations of good quality reservoirs. The aims of this study were: to characterize the chronostratigraphic depositional facies and distribution, to examine the affect of diagenesis on reservoir quality and to define those parts of the basin where potential reservoir sands are likely to be found. A detailed analysis of depositional facies in the Murta Formation and its basal McKinlay Member was based on drill core analysis and regional wireline log correlations. The application of non-marine sequence stratigraphic concepts to the wireline logs and core description data have led to the identification of 7 chronostratigraphic units. This data was used in constructing a series of schematic palaeogeographic and isopach maps of the study area to predict the changes in depositional styles with time and space. Six depositional facies were identified in the study area with overall fine-grained sandstones, siltstones and mudstones. These depositional facies include; distributary channels, shoreline, protected shoreline, deltaic mouth bars, tempestites and turbidites deposits. One particular unit, the transgressive systems tracts of the McKinlay Member, (Unit 7) is the best target for reservoir development because of relatively high proportion of pay sands, medium to coarse-grained sand sizes and good reservoir quality with low diagenetic affects. High stand systems tract units 2, 3 and 5 are considered to be secondary reservoir targets because of diagenetic affects and their fine-grained character. Units 1, 4 and 6 are not considered of economic value for oil production because of their extremely low reservoir quality and muddy lithologies. The main diagenetic affects on sandstones (quartzarenites) in the study area are quartz overgrowths, formation of authigenic clay (kaolinite), carbonate cement (calcite and siderite), formation of microstylolites (pressure solution) and dissolution of the framework grains to form secondary pores. The McKinlay Member of the Murta Formation in South Australia consists mainly of medium to coarse-grained sandstones of distributary channel origin (facies association 1). It has the highest recorded porosity and permeability (9.5% and 36.8mD respectively). Shoreline, protected shoreline, deltaic mouth bars and tempestites deposits, mainly from Jackson-Naccowlah Trend wells in Queensland, are of finegrained sandstones (facies associations 2, 3, 4 and 5 respectively). They have adequate average porosity (7%), but the formation of microstylolites and associated mica parallel to the bedding planes inhibited vertical permeability and has been recorded to be as low as 3.1mD. Turbidites in the central basin are characterized by extremely low reservoir quality (2.6% and 0.25 mD) and muddy lithologies of facies association 6 that are severely degraded by diagenetic effects.