| Popis: |
Monterey oils in the Santa Maria Basin onshore and offshore are generally low maturity oils because of low thermal exposure (relatively shallow burial for short times). They are generated as heavy high-sulfur oils because of the composition of the unusual Type II source kerogens which are exceptionally rich in organic sulfur (8–14%). These high-sulfur kerogens, which we designate Type II-S, appear to generate oil at significantly lower thermal exposures than ordinary Type II kerogens with less than 6% S. In contrast to common Type II kerogens which generate oil largely by cleavage at oxygen bridge groups and carbon-carbon bonds (eliminating CO2, H2O, and hydrocarbon fragments), Type II-S kerogens have an additional complement of weak bonds associated with bound sulfur. The preferential cleavage at weak sulfur linkages tends to produce larger fragments, leading to high initial amounts of asphaltenes, resins, and sulfur-rich aromatics together with smaller amounts of saturated hydrocarbons. Elemental analyses of Monterey kerogens and asphaltenes clearly support the concept that the asphaltenes are essentially fragments of kerogen cleaved sufficiently to be soluble or peptized in this heavy oil system. Solubilization of asphaltenes is attributed to their interactions with the abundant resins and aromatics. This sysstem is sufficiently stable under source and reservoir conditions to allow expulsion and migration for reasonable distance without significant fractionation or changes in gross properties. Initial asphaltenes have essentially the same atomic H/C ratios as the source kerogen but have significantlyy lower O/C ratios and slightly lower S/C ratios. The N/C ratios are nearly the same for kerogens and their derived asphaltenes. With increasing maturity the H/C ratios in asphaltenes decrease much like maturation changes in kerogen. The S/C ratio also decreases significantly. However, the O/C ratios (initially low in asphaltenes) and N/C ratios increase slightly due to material balance caused by other losses. These element composition and atomic ratio relationships allow some important inferences concerning source kerogen composition from analysis of asphaltenes alone. This can be useful when potential source-rock samples are not available for analysis and/or when a choice between alternate source-rock possibilities must be made. |
