Overpressure origin and its effects on petroleum accumulation in the conglomerate oil province in Mahu Sag, Junggar Basin, NW China

Overpressure characteristics and effects on hydrocarbon distribution in the Bonan Sag, Bohai Bay Basin, China

In situ overpressures in sedimentary basins are commonly attributed to disequilibrium compaction or fluid expansion mechanisms, although overpressures in shallow sedimentary sequences may also develop by vertical transfer of pressure from deeper basin levels, for example, via faults. Mafic sill complexes are common features of sedimentary basins at rifted continental margins, often comprising networks of interconnected sills and dikes that facilitate the transfer of magma over considerable vertical distances to shallow basinal depths. Here, we document evidence for deep sills (depths >5 km [>16,000 ft]) hosting permeable, open fracture systems that may have allowed transmission of overpressure from ultradeep basinal (>7 km [>23,000 ft]) levels in the Faroe–Shetland Basin, northeast Atlantic margin. Most notably, well 214/28-1 encountered overpressured, thin ( 58 m.y.). We instead suggest that transgressive, interconnected sill complexes, such as those penetrated by well 214/28-1, may represent a previously unrecognized mechanism of transferring overpressures (and indeed hydrocarbons) laterally and vertically from deep to shallow levels in sedimentary basins and that they represent a potentially underrecognized hazard to both scientific and petroleum drilling in the vicinity of subsurface igneous complexes.

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Overpressure transmission through interconnected igneous intrusions

Distribution, origin, and evolution of overpressure in the Shahejie Formation of northern Dongpu Depression, Bohai Bay Basin, China

Reevaluation of the origin of overpressure in the inter-salt shale-oil reservoir in Liutun Sag, Dongpu Depression, China

The origin of the overpressure in the northern Qaidam Basin has not been clearly understood, which has caused some difficulties in hydrocarbon exploration. Using a compaction study, we applied a modified acoustic-velocity and effective-stress diagram to identify the overpressure transfer in the study area. This phenomenon has not been discussed in previous studies. For the present study, we approximately calculated the magnitude of the transfer overpressure and analyzed the cause of the overpressured aquifer at the crest of the anticline in the study area. Our study indicates that the effect of overpressure transfer is very distinct, and the largest contribution to the total overpressure is 57%. The main media of overpressure transfer include vertical faults and lateral conducting layers. The vertical faults can connect deep overpressured strata, and the lateral conducting layers can connect overpressured strata at the top and wing of the anticline. During anticline formation, the crest fractures, and then the overpressured water in the anticline wing flows into the fractured crest and forms the overpressure compartment that prevents the charging of deeper natural gas.

Xiao Zhang

Papers

Identification and calculation of transfer overpressure in the northern Qaidam Basin, northwest China