Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253-6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at < 1600 m, but was replaced by dolomite at greater depths. Dolomite at similar to 1600-2700 m is Fe-poor (< 0.8 mol% FeCO3 and at similar to 4700-6200 m, Fe-rich (0.5-7.2 mol% FeCO3 Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated delta(18)O (+21 parts per thousand to +25 parts per thousand) and delta(13)C (- 6 parts per thousand to - 2 parts per thousand) values that generally decrease with depth. Dolomite has a wider variation in delta(18)O (+ 18 parts per thousand to + 30 parts per thousand) and delta(13)C ( - 8 parts per thousand to - 1 parts per thousand) values, which also generally are lower with increasing depth. However, the delta(13)C and delta(18)O values of dolomite from the uppermost - 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval. The decreasing dolomite delta(13)C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (less than or equal to 55 degreesC) with porewater of meteoric origin. Its delta(18)O values increased from similar to - 5 parts per thousand at the shallowest depths to - 0 parts per thousand at - 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from similar to 0 parts per thousand at - 1600 m to similar to + 7 parts per thousand at 3600 m ( similar to 100 degreesC). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose 6180 values decreased from similar to + 9 parts per thousand at similar to 4750 m ( similar to 120 degreesC) to similar to + 1 parts per thousand to + 2 parts per thousand by similar to 6200 m ( similar to 150 degreesC). Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rockwater ratios tended to be high during these processes. However, the upper - 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower 180 fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area. (C) 2003 Elsevier B.V. All rights reserved.