GEOLOGY, THERMAL MATURATION, AND SOURCE-ROCK GEOCHEMISTRY IN A VOLCANIC COVERED BASIN - SAN-JUAN SAG, SOUTH-CENTRAL COLORADO

The San Juan sag, concealed by the vast San Juan volcanic field of sou th-central Colorado, has only recently benefited from oil and gas wild cat drilling and evaluations. Sound geochemical analyses and maturatio n modeling are essential elements for successful exploration and devel opment, Oil has been produced in minor quantities from an Oligocene si ll in the Mancos Shale within the sag, and major oil and gas productio n occurs from stratigraphically equivalent rocks in the San Juan basin to the southwest and in the Denver basin to the northeast. The object ives of this study were to identify potential source rocks, assess the rmal maturity, and determine hydrocarbon-source bed relationships, Sou rce rocks are present in the San Juan sag in the upper and lower Manco s Shale (including the Niobrara Member), which consists of about 666 m (2184 ft) of marine shale with from 0.5 to 3.1 wt % organic carbon. P yrolysis yields (S-1 + S-2 2000-6000 ppm) and solvent extraction yield s (1000-4000 ppm) indicate that some intervals within the Mancos hale are good potential source rocks for oil, containing type II organic ma tter, according to Rock-Eval pyrolysis assay. Oils produced from the S an Juan sag and adjacent part of the San juan basin are geochemically similar to rock extracts obtained from these potential source rock int ervals. Based on reconstruction of the geologic history of the basin i ntegrated with models of organic maturation, we conclude that most of the source rock maturation occurred in the Oligocene and Miocene, Litt le to no maturation took place during Laramide subsidence of the basin , when the Animas and Blanco Basin formations were deposited, The timi ng of maturation is unlike that of most Laramide basins in the Rocky M ountain region, where maturation occurred as a result of Paleocene and Eocene basin fill, The present geothermal gradient in the San Juan sa g is slightly higher (average 3.5 degrees C/100 m; 1.9 degrees F/100 f t) than the regional average for southern Rocky Mountain basins; howev er, although the sag contains intrusives and a volcanic cover, the gra dient is significantly lower than that reported for parts of the adjac ent San Juan basin (4.7 degrees C/100 m; 2.6 degrees F/100 ft). Burial depth appears to be a more important controlling factor in the therma l history of the source rocks than local variations in the geothermal gradient due to volcanic activity, Interestingly, the thick overburden of volcanic rocks appears to have provided the necessary burial depth for maturation.