--> Emplacement of Sand Injections During Contractional Tectonics

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Emplacement of Sand Injections During Contractional Tectonics

Abstract

Intrusive geological bodies such as magma or salt have been widely recognized in contractional tectonic settings where they fill anticlinal cores or are intruded along thrust planes. However, much less is known about clastic intrusions that potentially intrude along and fill contractional structures. There has recently been a growing interest regarding the study of sand injection complexes and their relevance to hydrocarbon reservoir characterization. Giant exposures of sand injection complexes, including sandstone dykes and sills which cut through hundreds of metres of hydrofractured mudstones, have been described in detail in the Upper Cretaceous-Eocene succession cropping-out in the Panoche and Tumey Hills area (Central California). Here, the occurrence of a large volume of sandstones, organized as interconnected dikes and sills, importantly increases the connectivity and permeability within the host mudstone, thereby providing valuable pathways for fluid migration. In contractional settings, sand intrusion is in theory attributed to high pore-pressure conditions that develop because of the increase in compressive stress. The contribution provided by horizontal compressive stresses (tectonics), although hypothesized, is still to be substantiated by robust field observations. A detailed field survey, carried out in the Panoche and Tumey hills area, allows contractional tectonics-related sandstone intrusions to be recognized for the first time. These sandstones cut through the Upper Cretaceous to Eocene succession (Moreno and Kreyenahgen formations) and are observed to intrude along thrust planes, reverse fault planes and dilational jogs. These relationships allow us to ascertain the role played by contractional tectonics on emplacement of sandstone intrusions during the contractional stages which involved the Great Valley Sequence basin-fill succession. The recognition of such sandstone intrusions systems in contractional settings will allow refinement of reservoir connectivity models.