High-Resolution Seismic Stratigraphic Applications To Petroleum Exploration

Abstract

High STAND SYSTEMS TRACT (HST) AND CONDENSED SECTION (CS) The sequence stratigraphic cycle starts towards the end of a high stand in sea level as experienced under present day conditions. sediments accumulated during this period of the cycle are part of the highstand systems tract ([1ST)of Figs, 2 and 3. During the HST stage there is very little deposition on the outer Continental shelf and beneath the deeper water of the slope, as sediment accumulation consists Primary in a slow "rain" of microscopic shells of plankton and other fossils and wind transported sediments, The slow accumulation of sediments during this phase the cycle is "condensed" relative to time, and it is the condensed layer tire identified on high resolution seismic records by laterally continuous strong acoustic reflection and the attitude of overlying und underling reflector (Fig 1 and 5). The point in time at which sea level reaches its Maximum incursion shoreward is referred to as the "maximum flooding stage" (mfs of Figs 2 and 3). This occurs during the warmest portions of an interglacial stage, and the resulting sediment layers are characterized by an abundance of microfossils. A "sequence boundary" is located immediately above the condensed section provided no erosion has taken place, in which case the boundary follows the unconformity. High rates of sediment accumulation during the HST phase are limited to points of high fluvial discharge along the shoreline and the occasional delta which builds out onto the shelf. Such is the case at the mouth of the present-day Mississippi River. The HST portion of a cycle is completed when sea level drops sufficiently to place the shoreline along the outer portions of the continental shelf As the cycle progresses the shoreward areas become exposed and dissected by fluvial processes developing a coastal plain similar to the present-day areas of south Texas and Louisiana, SHELF MARGIN SYSTEMS TRACT (SMST) As sea level drops as a result of global climatic cooling and the buildup of continental ice sheets the shore line slowly migrates towards the outer continental shelf accumulating sediments seaward where supplied in a "prograding"fashion [Fig. 3), Those areas experiencing high sediment discharge rates on the mid to outer shelf areas develop shelf margin systems tracts (SMST), while elsewhere relatively little deposition takes place. The SMST depo-centers consists of lobe-shaped sedimentary complexes which tend to meander and switch position along the shoreline or shelf edge during this period. Upon reaching the shelf edge with a further drop in sea level sediments are dumped down the steeper inclines of the upper slope to form the lowstand systems tract components of Figs. 2 and 3. LOWSTAND SYSTEMS TRACT (LST) Upon reaching it's lowest elevation towards the peak of an ice age, sea level and the shoreline are located along the continental shelf edge. This is more than a hundred miles off the present Texas coast in approximately 500 feet of water.