Shallow-Marine Massive Sandstone Sheets - How Synsedimentary Seismic Events Enhance Reservoir Connectivity
The Middle to Upper Ordovician siliciclastic succession in Central Bolivia provides excellent exposures of up to 1 m thick sandstone beds produced by liquefaction-induced sediment remobilization. These fine-grained massive sandstones occur in shallow-marine nearshore facies that was deposited above storm wave base. Vertical to steeply inclined clastic dikes, which penetrate up to 1.5 m of the overlying sediment pile, feed into the basal parts of massive sandstone sheets. These dikes are interpreted as pathways used by liquefied sediment during upward-directed escape from a subsurface horizon. The relatively short lateral spacing of 5 to 25 m between the individual dikes initiated lateral coalescence of ejected individual sediment bodies. As a result, massive sandstone sheets formed and can be traced laterally over several kilometers. The massive sandstones contain elongated or S-shaped sandy, laminated fragments. Their outer form reflects squeezing showing that they have been deformed under shear stress during flow. The axis of maximum elongation of the fragments indicates local flow direction and mirrors paleo-inclination of the shelf. These massive sandstone sheets are probably products of seismic shocks, a process that could have a favorable impact on subsurface hydrocarbon reservoirs. This outcrop analog shows that adjacent non-liquefied sandstones often exhibit lenticular geometries whereas their massive counterparts are sheets and laterally persistent for kilometers. Furthermore, the clastic dikes feeding into the massive sandstones still enhance connectivity of a potential reservoir as they provide vertical pathways for fluids. Synsedimentary seismic activity therefore seems to have the potential of distinctly enhancing reservoir connectivity and should be thoroughly explored also for shallow-marine shelf environments.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas