Abnormal fluid pressures caused by deposition and erosion of sedimentary basins
Gravitational loading or unloading due to deposition or erosion in sedimentary basins is one of the most common mechanisms responsible for abnormal fluid pressures in geologic formations. This paper examines the integrated response of fluid pressures to both deposition and erosion using vertical one-dimensional numerical models. The modeling results show that vertical loading and unloading of sedimentary basins containing thick interbeds of low permeability can give rise to abnormal fluid pressure-depth trends that closely resemble those observed in the field. On a geological time scale, an under-pressured formation may become normally pressured or over-pressured and vice versa because of transient flow induced by vertical geologic movements. A case study in the Guymon area of Texas County, Oklahoma, shows that there are two underpressured reservoirs in this area. The burial curve constructed from geologic information indicates that this area has undergone multiple subsidence and uplift events. Based on stratigraphic information, a vertical geologic section of this area is represented by a numerical model consisting of two reservoir formations and two shale (confining) units. The numerical model is able to reproduce the observed pressure abnormalities based on the drill-stem test (DST) data. The hydraulic conductivities of the two shale formations are estimated to be 4.2 × 10 and 5.1× 10 m s from model calibration.q 1998 Elsevier Science B.V.
Regionally extensive emplacement of sandstone intrusions: a brief review
Large sandstone intrusion provinces (LSIPs) are described from three basins with contrasting tectonic settings, the North Sea Basin, the Faeroe‐Shetland Basin and the San Joaquin Basin. Hundreds of reservoir‐scale sandstone intrusions have been identified in each of these basins over areal extents of >500 km2, and with vertical extents of >1 km from their underlying source units. It is likely that these large intrusion networks were emplaced over geologically short time periods i.e. as a single hydrodynamic event, because of their highly interconnected geometry. Large sills are extensively intruded into the host claystones in each basin, and the presence and distribution of these intrusions demonstrates that pore fluid pressures in the source units may have been in excess of lithostatic at the time of remobilization and intrusion. Mechanisms that could bring source sand bodies of large volume and areal extent to a condition of supralithostatic pore fluid pressure are not well understood in the context of sandstone intrusion. Of the possible candidate mechanisms, shear‐induced liquefaction resulting from a large earthquake or bolide impact and geologically rapid pressure transfer from higher pressure cells are the most likely. The relative rarity of LSIPs worldwide is either a result of undersampling at outcrop or perhaps signifies that a rare combination of factors is required to form them. Once formed, they disrupt the sealing properties of the sequences they intrude, and this has major implications for fluid migration in these areas.
Fast seismic data compression based on high-efficiency SPIHT
Based on the set positioning in hierarchical tree (SPIHT) algorithm, fast seismic data compression based on the high-efficiency SPIHT (HESPIHT) algorithm is presented. To verify the validity of the algorithm, 10 seismic cross-sections are selected for the experiment. The result shows that the algorithm can reserve the quality of data better, meanwhile improving the efficiency of encoding and decoding.