Lower Paleozoic reservoir rocks generally have a predominantly carbonate lithology and are singularly lacking in organically rich dark shales. Attempts to trace the source for many such deposits have proven to be difficult because of the absence of routine techniques normally used in source rock evaluation. A good example is the occurrence of hydrocarbons in the Lower Ordovician carbonate strata of the Ellenburger Group. Very little information is available on the nature of the source rocks involved in generating the hydrocarbons found in the Ellenburger. One viewpoint remains untested, namely, that these hydrocarbons have been generated within the fine-grained carbonates of the lower Ellenburger rocks themselves and trapped in the upper porous portions of the formation. everal factors may be responsible for not accepting this hypothesis. Three of these are the following. (1) Do carbonates make good source rocks? Carbonates with algal or sapropelic matter make excellent source rocks; it also appears that the total organic carbon values of carbonates in ancient rocks could be as low as 0.2% for adequate generation. (2) With vitrinite absent, how can organic maturation in lower Paleozoic rocks be determined? Conodonts and acritarchs, as well as sapropelic organic matter, can be used to determine maturation with considerable accuracy. (3) Could hydrocarbons generate in rocks as old as the early Paleozoic and situated at great depths? Drilled data from several regions of the world support this viewpoint.

Recent field and theoretical considerations suggest that the source units for the majority of the lower Paleozoic hydrocarbon occurrences in carbonates, including those of the Ellenburger, are fine-grained carbonates, which are frequently in contact with the productive zones.

End_of_Article - Last_Page 147------------