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Geochemical and petrographic studies of source rocks in the region of the Malheur, Jordan and Andrews Resource Areas, southeastern Oregon, indicate that some strata may be locally capable of generating and perhaps expelling hydrocarbons. Wildcat wells to date, however, have not demonstrated the existence of commercial hydrocarbon production in or near the Malheur, Jordan and Andrews Resource Areas. Regional geology suggests source rocks may occur in small deposits dispersed throughout the resource areas and this basis three conceptual plays are proposed: Alvord Valley-Steens Mountain, Harney Basin and Vale basin. Geohistory modeling of these conceptual plays shows that the high heat flow characteristic of the area combined with Tertiary to Recent burial to over 1 or 2 km, depending on heat flow, apparently heated the hypothesized source rocks into the hydrocarbon generation window in the Miocene. Traps in these conceptual plays are considered to be related to fault truncation, stratigraphic pinchouts and secondary porosity related to the diagenesis of volcanic provenance rocks. Permeability is assessed to be generally poor in potential reservoir rocks because unstable volcanic glasses are a common component of the rocks and during diagenesis they have been altered to clays, zeolites and related materials which fill pores and reduce permeability. In conclusion, the resource areas are considered permissive for small to medium size hydrocarbon discoveries but are not considered favorable. Introduction This paper examines the organic geochemistry and petrography of potential source rocks and their geohistory in the Malheur, Jordan and Andrews Resource Areas, southeastern Oregon. These data and geohistory modeling are used to assess conceptual oil and gas plays with potential for hydrocarbon accumulations. These conceptual plays are formulated to include the geological environments in Malheur, Jordan and Andrews Resource Areas that appear favorable for hydrocarbon generation, accumulation and preservation. Conceptual plays are postulated hydrocarbon "accumulations 1 sharing similar geologic, geographic, and temporal properties such as source rock, migration pathway, timing, trapping mechanism, and hydrocarbon type" (Gautier et al., 1995). Play areas are geographic regions where the defining play concepts are considered valid. Following Barker et al., (1995) these conceptual plays are defined on the ongoing burial of Neogene source rocks in the extensional basins of the Basin and Range Province. As discussed in Barker et al. (1995), because of the sparse drilling, poor sample availability and lack of analyses, information on traps, reservoirs and other geologic details in these conceptual plays is largely speculative. Further, numerous wildcat wells have been drilled in the Malheur, Jordan and Andrews Resource Areas but these tests have resulted in no commercial hydrocarbon production (Olmstead, 1988). Thus, the conceptual plays presented in this study are unproven. The Malheur, Jordan and Andrews Resource Areas of southeastern Oregon are a subregion of province 18 of the USGS 1995 petroleum assessment (Barker et al., 1995) which encompasses eastern Oregon, western Nevada, and eastern California. The conceptual plays presented here are derived from that analysis of hydrocarbon potential which found low or no potential. The data available for that study have been augmented by additional source rock analyses from samples in wildcat wells and surface exposures in the area. Thus, the purpose of this study is to reexamine the hydrocarbon potential in the light of these new data. Acknowledgments I acknowledge the contribution to this study by: James Evans, U.S. Geological Survey, Spokane, Washington who supplied samples; Lanny H. Fisk, Consulting Geologist, who provided several references and his personal knowledge on the plays of the area; Terry Giesler from the U.S. Bureau of Land Management, Burns, Oregon, who arranged a field trip into the Andrews Resource Area; Phyllis Halvorsen, USGS, Menlo Park, California, who provided a gravity map and an interpretation of the Steens Mountain and Harney Basin areas; Mont Warner, Consulting Geologist, who provided geological notes and references on the geology of the Snake River Plain; and Dan Wermiel, Oregon Department of Geology and Mineral Industries, Portland, Oregon, for arranging to examine well records and sample well cuttings and core. Methods The well samples were collected from the Oregon Department of Geology and Mineral Industries (DOGAMI) in Portland, Oregon. The well samples derived from cuttings were cleaned of well additives and, if possible, picked for specific rock types thought to be representative of the depth interval at the depth indicated on the sample bag. Surface samples were collected from exposures of Miocene Alvord Creek beds along the eastern side of Steens Mountain by Barker and from the northern edge of the Malheur Resource Area by James Evans. Selected well and surface samples were prepared for Rock-Eval pyrolysis and vitrinite reflectance analysis using the method of Barker (1994). Rock Eval Pyrolysis Rock-Eval pyrolysis is a source-rock assay technique that involves heating small quantities (50-400 mg depending on carbon content) of pulverized rock and measuring the mass of hydrocarbon gases evolved as a function of temperature. The carbon dioxide evolved during heating is saved in a trap during heating and later analyzed to estimate the total organic carbon content of the sample. During the initial stages of heating, sorbed or free hydrocarbons in the sample are driven off and are recorded as the Si peak. With increasing temperature, the organic matter in the sample breaks down to hydrocarbons and CO2 .which are recorded as the S2 peak and S3 peak respectively, and other compounds which are not analyzed. Specific definitions for Rock-Eval data reports are: &! and S2 are the first and second peaks of hydrocarbon (HC) yield occurring during pyrolysis of the sample. S3 is the amount of C02 generated during pyrolysis; TOG is total organic carbon. Tmax is the temperature at which the S2 peak occurs during pyrolysis of kerogen. Derivative values from these basic measurements are the hydrogen index (HI) = (S/rOC)x100; the oxygen index (Ol) = (S3/TOC)x100; the genetic potential (S^ +S2) and the Transformation ratio = PI = S1 /(S1 +S2). TOG when used in these derivative values is reported as grams carbon (g C). The analytical results of Rock-Eval pyrolysis are unreliable if the TOG content of the rock sample is less than 0.5 weight-% (Peters, 1986; Bordenave et al., 1993). Furthermore, rocks with less than 0.5 weight-% TOG are probably incapable of expelling hydrocarbons and therefore are not source rocks. Samples with less than 0.5 weight-% TOG are included in the data tables but are not included in the interpretative plots. It must be emphasized that Rock-Eval pyrolysis only gives a semiquantitative estimate of organic matter properties during rapid heating to extreme temperatures under dry conditions which at best can only be considered a rough analog to natural conditions. There is a strong tendency by geologists to take the semi-quantitative results from this poor experimental analog and use the values to calculate what appear to be excessive volumes of hydrocarbons that could be generated from the organic matter. This study interprets Rock-Eval results from the broad trends of grouped data, ignoring outlier data and avoiding generated hydrocarbon volume calculations. Even with these limitations, the trends shown by Rock-Eval analyses, if checked against other analyses such as organic petrography and hydrous pyrolysis, can be a useful indicator of thermal maturation and petroleum generation potential. This philosophy has evolved from my experience and published discussions of the interpretation of Rock-Eval data by Katz (1983); Peters, (1986); Langford and Blanc-Valleron (1990); and Bordenave etal. (1993). The consensus is that Rock-Eval pyrolysis data and the measurement of total organic carbon (TOG) qualitatively evaluate the source rocks tendency to oil and gas generation, past and residual hydrocarbon generation capacity, and thermal maturity (Table 1). Besides these technical limitations of Rock-Eval, organic matter contaminants and naturally occurring oils and bitumens can also interfere with the ST and S2 values by increasing them. Organic drilling mud additives often increase the hydrogen index (HI) and TOG. Samples contaminated by particulate mud additives were detected by examination under a binocular microscope and cleaned by sieving, blowing on the sample to remove the lighter organic materials and selectively picking rock chips with tweezers. Rock-Eval pyrolysis is also influenced by migrated oil or bitumen. Such migration produces an ST peak greater than 2 mg hydrocarbon (HC)/g rock, an anomalously high transformation ratio and low T^ as compared to adjacent samples, and a bimodal S2 peak. The low T^ may also be related to weak S2 peaks resulting from low TOG values and not from organic contamination. Generally no oil staining or immature bitumen was observed in the Neogene age samples, so migrated bitumen or oil is assumed to not to be a factor in the Rock-Eval analyses. Table 1. Guidelines to the interpretation of Rock-Eval and Vitrinite Reflectance Results Based on Type II and Type III Organic Matter. Compiled from Peters, (1986), Langford and Blanc-Valleron (1990), and Bordenave et al. (1993), among others. TOC

u.s. geological survey open file report/open-file report

Organic geochemical analysis and thermochronologic modeling of potential petroleum source rocks in the Malheur, Jordan and Andrews resource areas, southeastern Oregon