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

摘要

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 253-6448m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements domiannte diagenesis. Calcite occurs at < 1600m, but was replaced by dolomite at greater depths. Dolomite at approx 1600-2700 m is Fe-poor (< 0.8 mol% FeCO_3), and at approx 4700-6200 m, Fe-rich (0.5-7.2 mol% FeCO_3). Magnesiste, anhydrite and halite are the final generally decrease with depth. has positively correlated delta~(18)O (+21 per thousand to + 25 per thousand) and delta~(13)C (-6 per thousand to -2 per thousand) values that generally decrease with depth. Dolomite has a wider variation in delta~(18)O (+ 18 per thousand to + 30 per thousand) and delta~(13)C (-8 per thousand to -1 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 approx 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 composition over this relatively small interval. The decreasing dolomite delta~(13)C values likely indicate an increase 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 preferen tial subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (<= 55 deg C) with porewater of meteoric origin. Its delta~(18)O values increased from approx - 5 per thousand at the shallowest depths to approx 0 per thousand at approx 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from approx 0 per thousand at approx 1600 m to approx + 7 per thousand at 3600 m (approx 100 deg C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re) equilibrated with porewater whose delta~(18)O values decreased from approx + 9 per thousand at approx 4750m (approx 120 deg C) to approx + 1 per thousand to + 2 per thousand by approx 6200m (approx 150 deg C). 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. Rock-water ratios tended to be high during these processes. However, the upper approx 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its trapid 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 ~(18)O 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.