Fluid Flow, Thermal History, and Diagenesis of the Cambrian-Ordovician Arbuckle Group and Overlying Units in South-Central Kansas

The Cambrian-Ordovician Arbuckle Group has historically been investigated for prolific oil production and is presently a candidate for carbon dioxide (CO2) sequestration in southcentral Kansas. Planning for oil production and CO2 sequestration necessitates an understanding of fluid flow and porosity evolution in the Arbuckle Group and overlying units. Integration of transmitted-light and UV epifluorescence microscopy, cathodoluminescence imaging, fluid inclusion microthermometry, and stable isotope analysis helps to establish a diagenetic history for the Arbuckle Group and to interpret fluid-flow conditions responsible for the reservoir character. The paragenesis organizes the complex diagenesis of the Arbuckle Group into earlyand late-stage sequences of porosity-enhancing and reducing events. Petrographic observations of mineral assemblages suggest multiple hydrothermal fluid migration events through the Arbuckle Group and overlying units via fractures and preferred stratigraphic horizons. The mineral assemblage, comparable to MVT deposits (baroque dolomite, megaquartz, calcite, sphalerite, and galena), supports the migration of hydrothermal fluids in the past. Findings suggest hydrothermal alteration of the extensively dolomitized Arbuckle Group resulted in dissolution and subsequent mineralization. Fluid inclusion homogenization temperatures from late-stage precipitates of the Arbuckle Group yield temperatures ranging from 93-131oC for baroque dolomite and 87-157oC for megaquartz, values higher than can be explained by burial conditions or an elevated geothermal gradient. Fluid inclusion ice melting temperatures yield salinities ranging from 16.3-20.4 wt. % NaCl equivalent for baroque dolomite and 3.1-6.0 wt. % NaCl 9 equivalent for megaquartz, values suggesting multiple fluids, as well as fluid evolution through time. Hydrocarbon fluid inclusions in the late-stage baroque dolomite suggest oil migration concurrent with hydrothermal fluid flow. Depleted δ 13 C and δ 18 O values provide evidence for a high-temperature basinal fluid source, as well as preferential flow of hot fluids in the upper portion of the Arbuckle Group, where a pore system related to paleokarst is overlain by a less permeable unit. Radiogenic strontium isotopic data, acquired from baroque dolomite (0.709080.71019) and calcite (0.71257-0.71716), supports fluid-rock interaction with siliciclastic material or basement rock at some point during the fluid migration history. Highly radiogenic calcite 87 Sr/ 86 Sr values, compared to those in baroque dolomite, suggest multiple sources of fluids responsible for late-stage cement and a transition from an advective fluid flow system to a vertical fluid flow system. The interpreted dependence of ancient fluid flow systems on major orogenies (Ouachita and Laramide) suggests modern fluids in the reservoir are not related to ancient systems, but are primarily sourced from topographic highs to the west and the east of the study area, as well as northern migration of basinal fluids into southern Kansas due to thermal expansion.