The Effect of Rapid Changes in Pressure, Temperature and Salinity Conditions on Methane Dissolution and Implication for the Volume of Accumulated Gas

Abstract

Formation of giant biogenic or mixed biogenic / thermogenic gas accumulations require several processes occurring in an order that maximizes volume of trapped hydrocarbons. They are: generation of large quantities of gas, development of efficient focusing mechanisms, and rapid formation of efficient shallow seals. During these processes a large quantity of gas is dissolved in formation water and rapid change of pressure / temperature conditions or salinity can lead to exsolution of gas. This study demonstrates that volumes of gas temporarily trapped in water may represent a significant contribution to large commercial accumulations. Several synthetic basin models designed to study the effects of erosion and sea level drop on accumulated gas volumes were built using Petromod®. Simulation results demonstrate that the amount of gas dissolved in water was significantly reduced after a rapid sea level drop. Up to 70 bcf of free gas per square kilometer of fetch area was added to the trap when the water level dropped by 1000 m as compared to the nominal case scenario where sea level was kept constant. In the case when water drop was followed by 500 m of erosion only 33 bcf/km² of free gas was added to the system compared to the nominal case. This means that rapid sea level drop added two times more of free gas to the trap than if sea level drop was followed by erosion. These findings can be further applied to explain volumetrics of known commercial accumulations of biogenic gas in general. An example is the Miocene discoveries in the Levantine basin of Eastern Mediterranean, which is believed to be of mostly biogenic origin. Conventional petroleum systems modeling based on biogenic gas generation kinetics failed to explain the large amounts of gas accumulated in the fields. Computed volumes were way below the reported 19 tcf of gas. Similar to the experiments on our synthetic models, the missing volumes could be explained by favorable conditions created by rapid sea level drop during the Messinian event, leading to gas release out of formation water and subsequent focused migration. Various paleo pockmarks observed on seismic slices in the pre-salt section provide further evidence of rapid changes of hydrocarbon phase/volume, intensified migration and “catastrophic” creation of large gas escape features on the Late Miocene seafloor.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016