LITHOLOGIC CORRELATION IN BEND SERIES, TEXAS. 



19 



If the top of the '"Bhxck lime" is taken at 

 520 feet in the generalized log, or a little above 

 the top of unit E, antl it is remembered that 

 the Seamaji well section is thicker and the 

 Rudd well section tliinner than that at Ranger, 

 the basis for the identification of oil horizons 

 that I have suggested in the generalized graphic 

 log can be readily worked out. If my 

 attempted identifications are for the most 

 part nearly right a conclusion bearing on the 

 theory of the origin of oil is suggested. Almost 

 all or all the horizons indicated correspond to 

 the highly glauconitic, phosphatic, sulpliidic, 

 coarsely sandy betls of the type that marks 

 the bases of units. That the minerals formetl 

 at these horizons are all the product of decay- 

 ing organic matter, ]>robably mostly animal 

 matter resulting from an unusual destruction 

 of life, seems almost certain. Then the pres- 

 ence of oil in these sands may be due not only 

 to the porosity of the sands but also to the 

 accumulation of organic matter directly in 

 association with them. 



Deductive considerations in themselves favor 

 this assumption, for the coai-sest sands will 

 naturally be deposited at the bases of units, 

 believed to represent the begiiming of trans- 

 gressions, and the coarsest sands are the ones 

 in which oil is generally assumed to accumu- 

 late. But as almost all these basal sands are 

 characterized by minerals believed to be due 

 to unusual amomits of organic matter the 

 relation between the coai-seness of the sand 

 and the origin of oil directly in it is inherent. 

 Local factors, such as cementation, may deter- 

 mine the exact position of the oil-bearing bed 

 in a sandy succession of beds, but the fact 

 remains that the jiresence of the oil and that 

 of the sand are independent effects of the 

 same cause, rather than that the presence of the 

 oil is the effect of the jiresence of the sand. For 

 that reason it makes little difference whether 

 or not in a sandy series like that in and adja- 

 cent to imit H, where the Ranger sand ])roba])ly 

 occurs, the jiositiou of the sand corresponds 

 exactly to one of the beds chosen as the base 

 of a unit or not. It is worth noting that the 

 coarsest sandstone from the Seaman well seen 

 imder the microscojie occurs at the base of 

 imit H, one of the beds suggested as eqtiiva- 

 lent to the Ranger sand, and that in the same 

 position in the Rudd well a considerable amount 



of coarse sand distinguished by a red ferrugi- 

 nous stain was found. 



Obviously an attempt to identify oil sands in 

 wells which have not been found to be produc- 

 tive, from general figures and statements as to 

 their positions, is a very speculative and arbi- 

 trary proceeding, more likely to express the 

 preconcepti(ms of the author than to form the 

 basis for conclusions. From the synthetic logs 

 of the individual wellsit is evident that the wells 

 penetrated numerous tliin sandstones that have 

 not been represented in the generalized log and 

 more sand and sandstone than could be rep- 

 resented in the .synthetic logs. It has been 

 impossible to indicate in the svnthetic logs many 

 sandstones that were neither isolated nor thick, 

 and the driller often fails to record slight 

 amounts of coarse sand or sandstone. On that 

 account the graphic logs can not be taken as 

 conclusive reference data as to the occurrence of 

 possible oil sands. But there are two such 

 sands that can be identified with considerable 

 certainty. One of them is Reeves's No. 3, of 

 which he says that it is at the top of the " Black 

 lime." This is taken to be the same as Matte- 

 son's h, of which he says that it is at the con- 

 tact of the Smithwick and Marble Falls 

 (presumably the "Black lime"), and Hill's I, 

 of which he says that it is immediately below 

 the top of the "Black hme." But my logs of 

 the Seaman well show that at the base of unit 

 D calcareous material was deposited, like that 

 at the top of unit E (the major portion of the 

 "Black ;ime") but more sandy; hence there is 

 good reason for believing that sand No. 3 

 corresponds with the basal bed of unit D. 



The other sand that can be rather definitely 

 identified is the Ranger sand (Reeves's No. 

 5) , of which both Reeves and Matteson say that 

 it is du-ectly overlain by a considerable thick- 

 ness of gray limestone. The intervals given by 

 Hill, Reeves, and Matteson all agree pretty 

 closely, and as the basal bed of unit G is the 

 only sand found in that part of the section 

 overlain by a considerable thickness of gray 

 limestone, the identification seems fairly trust- 

 worthy. The sand at the base of unit H, at 

 780 feet in the generalized log, is one of the 

 coarsest noticed in the section; and that bed 

 or any of the beds between it and the one at the 

 base of unit G may be the producing bed, but 

 it seems most likely that in the Ranger field the 



