The San Juan basin is an ovate, frontal, intermontane, structural basin slightly less than 20,000 square miles in area, encompassing almost 36,000 cubic miles of sedimentary rocks within the Dakota outcrop and above the Precambrian basement. This basin occupies most of northwestern New Mexico and a part of southwestern Colorado. The area of the basin was a southeastern shelf and positive area genetically related to the early Paleozoic Cordilleran geosyncline; an unstable marine shelf to sedimentational accessway southeast of the Pennsylvanian Paradox geosyncline bounded by the San Luis-Penasco (Nacimiento) coarse-clastic-producing highland on the north and east, and the Zuni-Defiance fine-clastic-producing lowland on the south and west.

The San Juan area was the locale of thick continental clastic deposition during Permian, Triassic, and Jurassic time, with only minor marine invasions from the southwest, west, and north. In late Cretaceous time, the area subsided gently as a highly oscillatory sedimentational shelf related to the Rocky Mountain geosyncline, with sediments moving into the shelf environment from westerly directions.

Localized Laramide folding and faulting of diverse trend preceded and accompanied Tertiary continental basining and subsequent epeirogenic rise of the basin to over 6,000 feet above sea level as some middle- to late-Tertiary volcanism affected the region. The structural basin subsided differentially to catch early Tertiary continental clastics derived from the tectonically active ringing highlands. From mid-Tertiary time to the present, successively older sedimentary rocks were exposed to erosion, ringing the youthful basin with breached anticlines, hogbacks, cuestas, and arcuate monoclines.

Five major layers encompass the critical compactional, orogenic, epeirogenic, and hydrodynamic activities and their intricate inter-relations responsible for commercial accumulation of oil in the two thickest and most complex marine layers. The major part of the oil in the San Juan basin has been found in stratigraphic, disconformal, and anticlinal traps of Mississippian and Pennsylvanian marine strata, and in stratigraphic, disconformal, anticlinal, fractural, and hydrodynamic traps in Upper Cretaceous strata. These strata encompass the maximum sedimentational variations within the marine shelf layers of the basin, and with no proved exceptions the oil is essentially indigenous to strata in which found, or originated in adjacent source shales.

The forces which conspired to create commercial accumulations of oil in Upper Cretaceous rocks are believed to have acted in the following sequence. 1. Compaction. 2. Closely subsequent Laramide stresses which caused folding and minor faulting. 3. Tertiary subsidence of the basin and concomitant tectonic upturning of strata complicated by major Tertiary normal and thrust faulting. 4. Meteoric water intake in the eroding Mesozoic sheet-like sandstones whose edges attained various altitudes on the inner (basinward) edges of the ringing Tertiary uplifts. The considerable overlap of compactional, orogenic, epeirogenic, and hydraulic events during Cretaceous and Tertiary time resulted in tilted oil-water contacts in certain anticlinal traps; fault accumulations by meteoric waters which alt red some of the shallower oils chemically; intrabasin oil accumulations in sealed lenses unaffected by fresh water drives and containing only traces of connate brines; and post-subsidence shifts of oil by brackish connate fluids within paralic and alluvial continental sandstones.

Careful sedimentational and stratigraphic analysis of the San Juan basin suggests that the major part of oil migration and accumulation in Cretaceous and Pennsylvanian strata had taken place prior to Tertiary basining, but that some late compactional migration in latest Cretaceous strata may have occurred as a result of increased load and regional dips genetically related to Tertiary subsidence and consequent increased overburden in the central part of the basin.

End_Page 366------------------------

Significant shifts of accumulation are believed to have taken place in certain types of reservoirs within blanket sandstones as a result of Tertiary and Quaternary hydrodynamic movement related to infiltering meteoric water.