Modified open-system hydrous pyrolysis

Abstract

As an alternative to classic batch hydrous pyrolysis experiments, this proof-of-concept study presents a version of open-system hydrous pyrolysis (OSHP) in which the design of the apparatus allows for a small, continuous flow of only liquid-phase water to pass through the reactor during artificial maturation. This allows volatile, liquid, and soluble materials to migrate freely through and out of the system, whereas insoluble mineral matter and kerogen remain within the reactor. This apparatus differs from previous OSHP designs in the ability to maintain true open-system conditions, thus minimizing any cracking or back reaction of the products with the residual kerogen.Using the OSHP apparatus, immature Anna Shale (Pennsylvanian age, Illinois Basin) containing Type II marine kerogen was matured artificially at experimental temperatures ranging from 300 to 360 °C for up to 72 h. Petrographically, the immature Anna Shale is dominated by a micrinitic groundmass, and contains lesser amounts of vitrinite (both collotelinite and collodetrinite), inertodetrinite, bituminite, solid bitumen, and alginite. Under run conditions, mean random reflectance increased from 0.46 to 1.81% for vitrinite and from 0.66% to 1.50% for solid bitumen. Alginite fluorescence changed from a bright yellow to dull red-orange and decreased in intensity until all fluorescence was lost in shales pyrolyzed at temperatures ≥350 °C. Rock-Eval Tmax increased from 425° to 520 °C, TOC decreased (e.g., from 12.5% to ∼9% at a run temperature of 360 °C), S1 decreased from 0.71 to ∼ < 0.1 mg HC/g shale, S2 decreased from ∼40 to <1.0 mg HC/g shale, and HI decreased from ∼300 to <10 mg HC/g TOC. These changes are consistent with the maturation of the immature shale through the oil window and into the gas window. With increased maturity, the shale showed a loss in mass: shorter runs (≤ 24 h) typically lost <10%, whereas longer runs (72 h) lost 12–20%. These losses were likely due to the loss of moisture, liberation of hydrocarbons and other gases, and depletion in total sulfur from 2.03 to ∼0.1 wt% (much of which was pyritic sulfur); higher mass losses may have included retention of material in the reactor, tubing, or frits. Petrographic observations revealed that pyrite framboids, which were abundant in the immature shale, decreased in samples pyrolyzed at temperatures >330 °C and showed evidence of alteration to hematite.The OSHP design was intended to mimic normal burial maturation. However, based on the significant depletion in sulfur (i.e., pyrite), the current design and run parameters may more closely replicate conditions associated with hydrothermal alteration, possibly those associated with intrusion of organic-rich rocks. It is also possible that the OSHP approach could lead to a better understanding of organic-mineral interactions and mobility of elements, especially trace elements, during maturation.