Origin and distribution of abnormally high pressure in the Mahanadi Basin, east coast of India

Drilling deep wells in the Mahanadi Basin of the east coast of India is highly challenging because of the variations in pore pressure in the Miocene formations. We have observed that the wells drilled in the northern part of the study area have more drilling hazards due to the presence of high pore pressure (modular dynamic test measurements of up to 18.5 ppg) when compared with wells drilled in the southern part of the basin. In the northern part, pore pressure prediction assuming disequilibrium compaction (DC) underpredicts by approximately 2–3.5 ppg when compared with observed pressures; however, in the southern part, pressure prediction matches the observed pressures in the drilled wells. Analysis of sonic velocity-density crossplots suggests that along with DC, some other secondary mechanism also plays an important role in generating excess overpressure in the northern part of the study area; however, the well data do not indicate the presence of an established secondary mechanism, such as fluid expansion, clay conversion, or cementation. The prime difference between the northern and southern areas is the presence of multiple canyon cuts in the northern part and the observation that very high overpressures occur below these canyon cuts. Hence, an attempt was made to ascertain the relationship between the presence of canyon cuts and the observed high pressure with the help of burial history modeling that incorporates the canyon cut features. Pressure estimation based on this approach closely matches the observed pressures in the drilled wells. This very high overpressure observed in the northern part is most likely generated by the combined effect of porosity rebound (due to overburden removal) along with persistence of overpressures that developed prior to erosion. This burial history modeling approach helps in recognizing and understanding the impact of erosional canyon cut events on generation of excess overpressure in the study area. Furthermore, effective stress methods that take secondary pressure generating mechanisms (unloading) into account are used to quantify the difference in pore pressure.