Based on data of outcrops

cores

image logs and thin sections

we analyzed the features of fractures in the shallow- to mid-depth reservoirs in Anpeng oilfield of Nanxiang Basin

compared them with that of the deep reservoirs

and determined the main factors controlling the development of these fractures.The study suggests that there are two types of fractures in the shallow-to mid-depth reservoirs in the Anpeng oilfield:the tectonic fractures and the diagenetic fractures.The former of high dip-angle dominates the study area with an average linear density of 0.42/m and a height of generally less than 0.40m(averaging at 0.25m)

which indicates that fractures are confined within the layers.Micro-fractures are very important channels for connecting the intergranular pores and intragranular dissolved pores

and enhance reservoir pore connectivity.Affected by horizontal tectonic compressions during the Early and Late Himalayan movements

three sets of tectonic fractures were formed in the study area

i.e.E-W

NE-SW and NW-SE striking fractures.The E-W fractures were developed better than the other two sets.The formation and distribution of these fractures in the oilfield were controlled by such factors as the tectonic stress field

lithology

porosity

permeability

layer thickness and structures.Under the same controlling factors

the fractures in the shallow- to mid-depth reservoirs of the field are thought to have the similar genetic types

occurrences and development characteristics to that of the deep reservoirs

but slightly lower degree of development than that of the deep reservoirs.