SOURCE AND COMPOSITION OF POTENTIAL END-MEMBER BRINES ASSOCIATED WITH SHALE GAS PRODUCTION IN THE APPALACHIAN BASIN OF CENTRAL PA
When plotted in the ternary NaCl-H2O-MCl2 system (where M=Ca2++ Mg2++Ba2++Sr2+), compositions of flow back brines from 55 Marcellus wells show a remarkably uniform composition of NaCl:MCl2 of 57.8:42.2, indicating that their compositions are controlled by equilibration with the shale matrix. Most brine compositions in the Appalachian Basin are consistent with this bulk shale matrix composition indicating that they originated by dewatering of bounding shales.
We recognize two other end-member brines. One group plots to the NaCl side of the shale matrix line and the other group plots to the MCl2 side. The former are high TDS brines that contain a component of Salina Salt, a Silurian-age evaporite (~70 wt.% halite, 15-20 wt.% anhydrite, 10 wt.% dolomite and 2-4 wt.% quartz). The latter group is a Ba-rich brine containing up to 15 kppm Ba and in central PA is sourced from the underlying Lower Devonian Oriskany sandstone. The Oriskany Fm. brine has reported TDS in excess of 300 kppm.
Our analysis shows that most published Oriskany brine compositions from the Appalachian Basin are actually mixtures of Oriskany Fm brine and Salina Salt. These brine mixtures all plot to the NaCl side of the shale matrix line. When Oriskany Fm brine is mixed with brine containing a Salina Salt component, the resulting brines are often saturated with respect to halite and depleted in sulfate due to precipitation of BaSO4 as a result of Ba mixing with Salina derived sulfate. This is typical of most published Oriskany Fm brine data from West Virginia and Ohio. By contrast, mixtures of Oriskany Fm brines with Marcellus RTW are saturated with respect to both halite and/or solid barium chloride dihydrate (BaCl2.2H2O). These mixtures plot to the MCl2 side of the shale matrix line.