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Continental-Scale Drainage and Basin-Scale Petroleum Systems — How Do They Interact?

Robinson, Paula *1; Macdonald, David 1
(1) Geology and Petroleum Geology, University of Aberdeen, Aberdeen, United Kingdom.

The pod of active source rock that lies at the heart of a petroleum system is limited to part of a single basin. In continental settings, the drainage network that supplies the reservoir sands can be on the scale of 35% of a continent (e.g. Amazon) and feed more than one basin; in such cases the reservoir element of the petroleum system can be largely independent of the basin. In strike-slip systems, a single river can feed several basins sequentially through time, in which case the local behaviour of the depositional system is influenced by the same tectonic processes that created the basin and controlled deposition and maturation of the source rock. The composition and reservoir quality of the sand being delivered to a strike slip basin by a large river system is, however, independent of intra-basinal processes and depends on the geology and scale of the catchment.

This paper presents a study of sediments from the modern Colorado River, from the Pliocene palaeo-delta of the Colorado in the Fish Creek-Vallecito Basin (Anza Borrego, southern California), and from the Los Angeles Basin where palaeo-Colorado deposits have been postulated. We use heavy mineral analysis to deduce the origin of the various sediment families. There is a clear petrographic distinction between locally derived sands and incoming Colorado sands in the Fish Creek-Vallecito Basin, however the distinction is less clear in the Los Angeles Basin. We demonstrate that the mineral piemontite is present as a detrital phase in the Sespe (Oligocene-Early Miocene) and the Topanga (Middle Miocene) formations of the Los Angeles Basin, with the most likely source being the palaeo-Colorado River. This will be tested with a detailed microprobe study of detrital piemontite from other localities in SE California and SW Arizona.

The exploration implications for southern California will be discussed, focusing on distribution of high-quality reservoir sands via the palaeo-Colorado. We also draw more general conclusions for other small basins that interact with large fluvial systems on other strike-slip margins.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California