Palaeohydrology of hydrocarbon maturation, migration and accumulation in the Dead Sea Rift H. Gvirtzman and E. Stanislavsky Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ABSTRACT It is proposed that hydrocarbon maturation, migration and accumulation in the Dead Sea rift, which took place during the last 3±6 Myr, were signi®cantly affected by density-driven groundwater ¯ow, from the rift westwards through deep aquifers. To test this hypothesis, a quantitative feasibility study was carried out using numerical modelling, which assesses the effects of structural evolution, groundwater ¯ow, heat transport and salinity redistribution on hydrocarbon migration. Results indicate that a signi®cant fraction of hydrocarbons have escaped to the land surface within the graben. Oil that has been forced westward into the rift ¯anks with the migrating brine is still moving. However, gas that has been forced westward has been trapped and forms the three small commercial gas ®elds: Zohar, Kidod and Haqanaim. A signi®cant portion of the oil has biodegraded forming heavy oils and asphalts due to subsequent ¯ushing by meteoric water. Simulations indicate that ¯ow-®eld con®gurations of both groundwater and hydrocarbons have changed during the structural evolution of the rift, illustrating the interrelationships between basin formation, palaeohydrology and hydrocarbon reservoir formation. INTRODUCTION Groundwater ¯ow plays a major role in a wide range of geological processes, such as hydrocarbon migration, ore deposition, upper-crustal heat transfer, earthquakes, diagenesis and metamorphism (Bethke & Marshak, 1990; Ingebritsen & Sanford, 1998). This study focuses on the in¯uence of groundwater ¯ow on the maturation, migration and entrapment of hydrocarbons. Initially, the thermal effects of vertical groundwater movement may in¯uence the position of the temperature `window' that favours the maturation of organic matter into hydro- carbons (Person & Garven, 1992; Person et al., 1995). Subsequently, oil tends to migrate mostly in the direction of groundwater ¯ow over long distances, as far as hundreds of kilometres (Garven, 1989; Bethke et al., 1991). Finally, hydrocarbon entrapment and accumula- tion depend on the geometry of the groundwater ¯ow ®eld (Hubbert, 1953; Dahlberg, 1994). In fact, hydrocarbon migration, groundwater ¯ow, solute transport and heat ¯ow all depend on the structural evolution of the basin, as is illustrated in this study, using the Dead Sea rift as an example. Numerical models have been used extensively in recent years to quantify basin transport processes. These models serve as a major research tool because of their ability to represent a complex basin geometry with heterogeneous and anisotropic porous media properties, and because of their ability to solve the coupled and nonlinear equations of variable-density groundwater ¯ow, heat transfer and multicomponent reactive mass transport. Using this technology, hydrocarbon migration and accumulation can be quantitatively reconstructed, and sensitivity of various parameters can be tested. Examples of such numerical investigations in a dynamic geological frame- work can be found in Willet & Chapman (1989), Garven (1989), Burrus et al. (1992) and Hayba & Bethke (1995), among others. In this study, the Dead Sea basin serves as a case study for applying hydrological modelling to evaluate the effect of continental rifting on the palaeo- groundwater ¯ow regime and, thereby, on hydrocarbon maturation, migration and accumulation. It should be noted that the Dead Sea rift is a relatively poor basin in terms of its hydrocarbon potential. Only a few asphalt and oil shows and small commercial gas ®elds are found in the rift and in its vicinity. Despite the small number of hydrocarbon shows, we believe the Dead Sea rift represents a fascinating basin for hydrodynamic study because of its well-documented geology, stratigraphy, tectonic history and geochemistry. Correspondence: Haim Gvirtzman, Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. Fax: 972 2 5662581; e-mail: haimg@vms.huji.ac.il Basin Research (2000) 12, 79±93 # 2000 Blackwell Science Ltd 79