| L Gasfield is a 100 billion cubic meters gasfield discovered in the Miocene of Yinggehai Basin, and its gas
bearing formation is a typical low-permeability sandstone reservoir. In order to provide a basis for the quantitative
prediction of reservoir fractures in L Gasfield and the next exploration and development, the fractures of different scales
such as imaging logging, drilling cores and thin sections in Huangliu Formation are studied. Combined with the test results
of whole rock mineral analysis and core porosity and permeability analysis, the development characteristics and main
controlling factors of fractures in low-permeability sandstone reservoirs were clarified. The results show that: (1) The
structural fractures account for more than 80% of fractures in the study area, of which shear fractures are mainly
developed, followed by tensile fractures, and the fracture dip angle is mainly low angle and horizontal (accounting for
more than 65%). Imaging logging images show that the fractures are mainly composed of parallel, reticular, conjugate and
Y-shaped combinations, and some fractures are partially or fully filled by calcite and argillaceous materials. (2) The main
influence factors on fracture development include stratum curvature, fracture position, rock mechanical parameters, matrix
porosity and permeability, lithology, and thickness of single sandbody. Fractures are highly developed in areas with large
stratum curvature, in fracture-developed areas, and in rocks with low granularity and high brittleness. Hence, the fracture
development degree is negatively and exponentially correlated with single sandbody thickness, but when the single
sandbody thickness is greater than 4.7 m, the correlation between the two weakens. With the increase in matrix porosity
and permeability, the fracture development degree first increases and then decreases. When matrix porosity is 8%-10%
and matrix permeability is (1.3-1.5) ×10-3 μm2, structural fractures are most developed. The fracture development degree
is affected by rock brittleness and strain energy, which shows that with the increase of Young’s modulus of rocks, the
fracture development degree first rises and then declines. (3) Since Neogene, Yinggehai Basin has undergone rapid
sedimentation, with strong undercompaction in the deep layer, widely developed diapir structures, and weak strata arching
together to produce unique low angle fractures in the thin sandstone layer sandwiched by thick argillaceous rocks in the
Huangliu Formation. |
