Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. The retort contains a fragmented permeable mass of particles comprising kerogen and various inorganic carbonates. A processing zone is established in the fragmented mass of formation particles and advanced therethrough. Off-gas, including carbon dioxide from carbonate decomposition, is withdrawn from the retort. The carbon dioxide content of the withdrawn off-gas, being a function of the processing zone temperature, is then determined. The determined carbon dioxide content of the withdrawn off-gas is then compared to values of carbon dioxide content of off-gas predicted as a function of processing zone temperature.

Abstract: A method of determining the depth of occurrence of a chemically aggressive bed in the course of drilling a well with the use of a drilling mud, wherein there is measured preliminarily the value and direction of the redox potential of the drilling mud injected into the well, whereupon the walls of the well are stripped of the mud cake and an additional portion of the drilling mud featuring its redox potential differing as to the value and direction from the redox potential of the drilling mud situated in the well, is prepared.

Abstract: A method is provided for evaluation of shallow electrical anomalies to determine which are likely to be caused by seepage from a hydrocarbon reservoir at depth. The method involves geochemical lithological, and petrographic analyses of anomalous sediments to determine whether these sediments contain certain features characteristic of seepage-induced anomalies. These features include the following:(1) accumulations of seeped (non-indigenous) hydrocarbons;(2) accumulations of carbonate cements that have .delta..sup.13 C values between -10 and -58.degree./oo;(3) pyrite accumulations in a shallow, porous host rock where the pyrite is distributed within fractures or between original grains with a disseminated or cement-like texture, and where it is not associated with kerogen or coal and is not framboidal in morphology;(4) close proximity of said pyrite and carbonate accumulations; and(50 anomalous (high) induced polarization (I.P.