No abstract

Geophysical methods, including gravity, magnetics, refraction and reflection seismic, together with drilling have revealed the presence of a steep, fault-bounded trough of Cretaceous age in the Boobyalla Plains area, northeastern Tasmania. The trough is at least 500 m deep and contains the first proven Cretaceous sedimentary sequence from onshore Tasmania in the Boobyalla Sub-basin. The Boobyalla Sub-basin is the southeastern extremity of the Bass Basin and is bounded by faults having NW-SE, N-S and NE-SW trends.These Late Cretaceous sediments consist of poorly sorted boulder conglomerate, often containing dolerite boulders several metres in diameter, pebble conglomerate and poorly sorted ferruginous sandstone. Clast lithologies are variable but reflect local derivation. Away from the trough margins the infilling sediments become finer grained with conglomerate becoming a less prominent part of the sequence. The coarse-grained sequences are interpreted to have been deposited rapidly in close proximity to a fault scarp.The sediments are biostratigraphic equivalents of and represent a proximal (near-source) facies of the Eastern View Coal Measures, which were encountered in Durroon 1, drilled in the Bass Basin and some 60 km northwest of Boobyalla Plains. A minor volcanic episode probably affected this section of the Bass Basin about 100 Ma B.P. This may be related to tectonic disturbances and could be responsible for an unconformity at the base of the Eastern View Coal Measures indicated by offshore seismic information.

In the Halverson Field area of the eastern Powder River Basin, oil is trapped in porous Minnelusa reservoirs by the Opeche Shale. This study indicates that the Opeche can be divided into two vertical units which were deposited in different environments and that the nature of the contact between the Minnelusa and Opeche is different for the two environments. The lower Opeche was deposited in a channel-fill environment and is in unconformable contact with the Minnelusa, whereas the upper Opeche was deposited in a marine or marginal marine environment and is in conformable contact with the Minnelusa. A paleo-high controlled the area of major facies change. The different environments of the Opeche should lead to different mapping techniques of the two shale units in the exploration for new Minnelusa oil fields.

South Cole Creek field, located in the southwest Powder River Basin of Wyoming has produced 11 million barrels of oil from the Dakota Sandstone. Detailed mapping of rock and reservoir properties including sandstone isopachs, initial potentials, cumulative production, and porosity-permeability data, reveals the existence of channel sandstones with excellent reservoir properties which are surrounded by marine rocks of poorer reservoir properties. The areal extent of the accumulation is controlled by factors other than structure and stratigraphy, as water-bearing channel sandstones exist lateral to and up-dip of producing channel sandstones. By comparing the known accumulation to the theoretical model for fluid flow, it can be shown that hydrodynamics has played an important role in modifying the accumulation at South Cole Creek field.

No abstract