(1) appearance; by noting the first occurrence of new forms or different as- 

 semblages; (2) population counts; by counting a given number of individuals 

 (200 is generally sufficient) of a certain group of microorganisms and plotting 

 the percentages of each genus or species in histogram form. It is sometimes pos- 

 sible to detect significant changes within that group which may be used as cor- 

 relation points; and (3) paleoecological analysis; the ratio of plant microfos- 

 sils to brackish or marine animal microfossils may be used as an aid in inter- 

 preting depositional environments. In some studies it may be advantageous to 

 revert to all three methods. 



Favorable Areas for Oil Exploration 



Oil is found generally rimming ancient depositional basins; seldom is it 

 found in the center of these basins. It is, therefore, important for the petroleum 

 geologist to interpret the paleoecological or ancient environmental conditions in 

 the area under exploration. Microfossils furnish excellent means for recognizing 

 these environments. In general, the presence of only spores and pollen, or Pedi- 

 astrum or Charophyta or fresh-water ostracodes in a sample would indicate that 

 the sediment was probably a continental deposit or was laid down close to an 

 ancient shoreline. On the other hand, a sample containing mostly hystrich- 

 ospherids or scolecodonts would suggest a brackish to marine environment. Ma- 

 terial containing foraminifera, discoasterids, radiolarians, chitinozoans, silico- 

 flagellates, echinoid or coral fragments would indicate an offshore or marine 

 environment. A study of bottom samples from the Gulf of Mexico and from 

 the Atlantic Ocean has shown a decrease in plant microfossils away from the 

 shore, an increase of the hystrichospherids in the brackish to marine environ- 

 ments, and a steady increase in the microforaminifera seaward. The results of 

 this study can be applied to ancient basins. 



If a series of composite samples from a particular time-rock unit can be 

 taken from several different boreholes, it is possible to approximate the position 

 of ancient shorelines. To do this, the specimens from each particular group 

 of microfossils — spores and pollen, hystrichospherids, microforaminifera, etc. — 

 are counted in each sample. Isobotanical lines are drawn on the abundance of 

 spores and pollen per gram of sediment and isozoological lines are drawn on 

 the abundance of hystrichospherids, microforaminifera, or other groups of ani- 

 mal microfossils per gram of sediment. 



The isobotanical lines parallel the approximate position of the ancient shore 

 and the general trend of oil fields. In addition, those isobotanical lines repre- 

 senting the greater abundance of spores and/or pollen are near the loci of oil- 

 bearing areas, whereas, the isozoological lines, indicating the greater abundance 

 of hystrichospherids or marine animals, are farthest removed from the oil-bear- 

 ing areas. To determine the direction of the shoreline, it is necessary to study 



205 



