Origin of enigmatic sand injectite outcrops associated with non-tectonic forced-folding structure in Bahrah area, northern Kuwait Bay

摘要

This is a quantitative and qualitative study for the first time linking surface with subsurface geology to understand the occurrence and origin of enigmatic sand injectite outcrops in two sites within the Bahrah coastal plain area, northern Kuwait Bay. Field work observations and aerial photography were used to describe and characterize the surface geology and stratigraphy of the study area. Thirty-one rock samples were obtained for petrographical, fluid inclusions and strontium isotopes analyses. 3D seismic data were also used to assess the subsurface geology of the area. The petrography and microthermometry of fluid inclusions were used to understand the origin of fluids and gases associated with the formation of these sand injectites. We found that these surface geological features represent surface manifestation of subsurface geological and biological activities/processes. Therefore, we proposed a near-surface complex focused fluid injection mechanism/model responsible for the formation of the sand pipes. The model consists of five episodes: Episode-1 represents the development of near-surface geobody (? Paleoshoreline) sealed by a shale layer, Episode-2 represents the entrapment of hydrocarbons that migrated laterally up-dip within geobody, interacting with mixed seawater intrusion and fresh groundwater; Episode-3 represents the effect of biodegradation biological processes of the entrapped hydrocarbons generating CH4, CO2 and H2S gases; Episode-4 represents overpressure build-up due to gas generation exceeding pore volume of geobody and forming non-tectonic forced-folds and polygonal to semi-circular joints/fractures network; and Episode-5 represents overpressure release triggered by external factor(s) (seismic activities?) facilitating gases, water and sand remobilisation through weakened zones of natural fractures.