Thermal Maturation Model of Late Proterozoic-Paleozoic Amadeus Basin, Central Australia: ABSTRACT

Abstract

The thermal maturation of the Amadeus basin, central Australia, was modeled using a modified version of Lopatin's time-temperature index. The late Proterozoic-Paleozoic basin presented numerous difficulties: (1) the absence of vitrinite in pre-Silurian strata (about 75% of the total section), (2) two major orogenic events that markedly deformed the basin, (3) moderate to extreme (25,000 ft, 7,620 m) amounts of surface erosion since the Carboniferous, and (4) a paucity of data (27 wells in an area equal in size to Oklahoma). The presence of possible giant oil and gas fields (from Ordovician shales as source) and significant gas discoveries in Proterozoic rocks made the study of special interest. Assuming a constant geothermal gradient of 1.65°F/100 ft to be representative of the basin, the amount of surface erosion was found to be the most significant factor controlling distribution of thermal facies. Interval-transit times in shales and palinspastic reconstructions of sedimentary thicknesses were used to estimate amounts of surface erosion and missing section associated with unconformities. An estimated 25,000 ft (7,620 m) of section has been removed along the northern margin of the basin, with approximately 8,000 to 9,000 ft (2,438 to 2.743 m) absent in the vicinity of the major oil and gas fields. Surficial erosion in both areas is the result of uplift associated with the Alice Springs orogeny. The Amadeus basin is an excellent example of a generation/migration/accumulation system favorable for commercial reserves of petroleum. The formation and accentuation of large anticlinal traps in Cambro-Ordovician sandstones slightly predated deep burial and strong oil generation in the Lower Ordovician Horn Valley Siltstone (Early to Late Carboniferous). The late Proterozoic Bitter Springs Formation, another possible source, was generative in the late Proterozoic to Early Ordovician. Suitable traps were also extant at this time. The thermal history of the Amadeus also demonstrates that maturation essentially halts if burial temperatures are substantially decreased by erosional unloading. In this manner sediments may remain deeply buried for long periods without undergoing substantial thermal alteration. The Amadeus is a very mature basin, with 47% of the strata being overmature (prospective for gas only). Hydrocarbon liquids are most likely preserved in the north-central part of the basin, as shown on a depth to liquid-limits map. End_of_Article - Last_Page 441------------