PROTEROZOIC SOURCE ROCKS - SEDIMENTOLOGY AND ORGANIC CHARACTERISTICS OF THE VELKERRI FORMATION, NORTHERN-TERRITORY, AUSTRALIA

The Velkerri Formation (similar to 1.43 Ga) in northern Australia is o ne of the organically richest Proterozoic successions in Australia and over the last 10 vr has been a target for oil exploration, This forma tion is predominantly a glauconitic marine shale deposited in restrict ed to anoxic conditions in a deep-water pro-delta slope to basin envir onment. The Velkerri Formation is composed of superimposed sediment co uplets made up of laminated black organic-rich mud shales [total organ ic carbon (TOC) similar to 4-6%] and laminated gray-green organically lean mud shales (TOC <2%). The couplers stack at scales ranging from c entimeters to meters, Of particular interest are three black organic-r ich mud shales in the middle Velkerri Member, tens of meters thick and correlatable over much of the drilled area of the basin, Their high o rganic contents reflect changes in orgallic productivity vs. the rate of sediment supply, and not fluctuating oxygen levels in the bottom wa ters or changes in water depth. Extractability and Rock-Eval data show more efficient expulsion of hydrocarbons from the upper two of the th ree organic-rich units in the middle Velkerri. Evidence also exists fo r local, intraformational migration of hydrocarbons into siltstone lav ers, Nevertheless, the middle Velkerri has very low permeability and p orosity, and still retains a significant fraction of its generated hyd rocarbons, especially in the lower portions of the middle Velkerri whe re a volcanogenic provenance led to the early blocking of pore throats by clay diagenesis during early burial. This study emphasizes the hig h quality of Middle Proterozoic source rocks for petroleum exploration in northern Australia, The main problem is finding traps that can pre serve hydrocarbons over er the long periods of time since the rocks be came charged with hydrocarbons.