Page 120 - Geology and Geochemistry of Oil and Gas
P. 120
92 CRUDE OILS
Fig. 5.1. Relationship between pressure gradient and percent of salt water in water–oil mixture (after
Trash and Brown, 1965, p. 8, chart 1). If SG ¼ 1.07, then the pressure gradient ¼ 1.07 0.434 ¼ 0.465.
Pressure gradient of oil+salt water mixture ¼ [(%salt water/100) (water gradient)]+[(%oil/100) (oil
gradient)]. For example, in the case of 50:50 oil–water mixture, if oil is 421API (0.354 psi/ft pressure
gradient) and specific gravity of water is 1.07 (0.465 psi/ft pressure gradient), then the pressure gradient of
a mixture ¼ (0.50)(0.354)+(0.50)(0.465) ¼ 0.41 psi/ft.
Midcontinent type of crude oil is mixed base and is found in Kansas, Oklahoma,
all of Texas except the Gulf Coast area, northern Louisiana, and Arkansas. It in-
cludes also eastern Colorado, parts of New Mexico, and Arizona.
The Gulf Coast type of crude is asphaltic and naphthenic in nature and is found in
the area lying in southern Louisiana and southern Texas.
Tissot and Welte (1978) classification of crude oils is presented in Fig. 5.3.
Botneva (1987, in: Eremenko and Chilingar, 1996, p. 62, 65) studied more than
400 crude oil samples in Russia and identified nine types. Six of these have been
subdivided according to their gasoline fraction composition and three, devoid of
these fraction, by the paraffin/naphthene and naphthene/aromatic ratios. The fol-
lowing chemical types (on the basis of paraffin/naphthene ratio) were identified:
Type I. Paraffin (paraffin/naphthene ratio 41).
Type II. Paraffin-naphthene (paraffin/naphthene ratio of 0.99–0.7).
Type III. Naphthene-paraffin (paraffin/naphthene ratio of 0.69–0.5).
Type IV. Naphthene-paraffin-aromatic (paraffin/naphthene ratio of 0.49–0.41).
Type V. Naphthene-aromatic (paraffin/naphthene ratio of 0.4–0.3).
Type VI. Naphthene (paraffin/naphthene ratio of o0.3).
Type VII. Dominance of paraffin-naphthene hydrocarbons (paraffin-naphthene/
naphthene-aromatic (PN/NA) 42).
