Vitrinite reflectance and maximum heating of the Lower Jurassic Inmar Formation in southern Israel

Maturity of sedimentary organic matter records the maximum heating experienced by the rocks. Vitrinite reflectance (%Ro) measured in 115 samples of the Lower Jurassic Inmar Formation in 21 boreholes in southern Israel range between 0.32 and 0.67 %Ro, corresponding to undermature to early mature levels with respect to oil generation. Lateral distribution of the measurements indicate a N-NE-trending elongated zone of maximum %Ro, and probably maximum heating for the Inmar Formation, approximately between the Gerar 1 and Shezaf 1 wells. In general, the %Ro measurements appear to correlate with the present temperatures in the wells studied. However, these temperatures are too low to account for the level of the %Ro measured, hence the %Ro measurements reveal higher past than present temperatures. The data obtained suggest a moderate regional cooling from the maximum heating to the present temperatures in the Inmar Formation by ca. 10-30 °C over most of the study area. The timing and geological context of the maximum heating are uncertain, but probably predate the Late Oligocene-Early Miocene regional erosion.