## Observation and Statistics of Structural Fractures

It is a meaningful and indispensable step for quantitative prediction of fractures to make statistics of the structural fractures on the core-rock samples. They will serve as the check data for the final result. The method employed to count fractures is as follows: fracture density refers to the total length of fractures on each unit of surface area for the core-rock, that is,

where D = fracture density.

?Li is the total length of structural fractures on the surface area of the core-rock. S is the surface area of the core-rock:

r stands for the radius of the core column; and L_{C} is the length of the core for statistics.

For those cracked core-rocks, we employed the method of Van Golf-Racht (1982) to get the volume fracture density:

d is the mean diameter of the core cracked rocks, that is d= (a+b+c)/3, while a, b, c stand for the long, middle and short axial of the samples respectively.

Generally, 3 to 5 core samples for each well are selected from different depths and the results shown in the sixth column of Table 4 are the results of weighted mean for these samples.

As revealed from the observation and statistics of the core-rocks, tensional-cracking is the dominant type, while the shear cracking is the secondary one. Based on statistic data, four areas of high fracture density are apparent: area around O8 and O26, area around O11, area around O5, and area around X3 (well locations shown in Fig.1b and Fig.3b). The distribution of the four areas trends NNE-SSW in a right-lateral echelon form, closely parallel to the fault system in the region. On the other hand, the low density areas are mainly located around O12 and around O30, with the lowest density of 0.0235/cm.

Petroleum exploration and production proved that the four high density areas are the main hydrocarbon productive zones, reinforcing the necessity for the prediction of the structural fractures in Olituoz area.