Rising Mud Diapirs and Their Thermal Anomalies

Abstract

Mud diapirs in the South Caspian Basin are noted for being massively gas-charged, for having extremely low temperatures compared to regional values, for having cold to ice-cold frothy mud and water emissions from gryphons and salses despite originating from several kilometers depth (as judged by mineral contents of the waters), and for having inferred heat- flux values which can be an order or magnitude or more higher than those inferred regionally.

Using both a self-consistent geometric evolution code for diapirs and sediments, together with a dynamical evolution code, GEOPETII, for sediment basins, this paper accounts for the observations in terms of a low thermal conductivity of mud diapirs relative to regional values. The consequence is to cause heat to defocus away from the diapiric base, thereby causing a higher temperature than in sediments regionally at the same depth, and to cause heat to focus towards the diapiric crest thereby providing a low temperature compared to regional ones. For the Abikh diapir the crestal domain isotherms can be about 2–3 km deeper than regionally. In addition, the higher diapiric basal temperature promotes more rapid conversion of kerogen to oil, and of oil to gas, which not only aid in the gas-charging of the mud diapir and which lowers the thermal conductivity even farther, but which also contributes to the buoyant rise of the mud diapir in the last 5–10 million years, coincident with massive sedimentation of about 10 km in the South Caspian Basin during the same time period.

The dynamical basin analysis model, GEOPETII, also includes exsolution of gas from oil and water at low temperatures and pressures, which cools the diapir even more by direct adiabatic gas expansion and by gas-charging of crestal pore space, which again lowers the diapiric thermal conductivity.

Numerical estimates indicate that the thermal influence of the Abikh diapir extends laterally about 2–4 diapir diameters into surrounding sediments near the crest. Near the diapiric base the thermal influence on hydrocarbon generation is profound, whereas sediment formation upturning distortions occur out to about six diapir diameters for all formations from Cretaceous through to Holocene, indicating a major influence of the diapir on gas and oil accumulations in and around the diapir.