Analysis Of Fractured Reservoirs In Ordovician In Tazhong Area, Tarim Basin

Numerous and various types of fractures are found in Tazhong area, includingunstructral fractures (diagenetic fracture and horizon stylolite) and structural fractures.According to different types of fillings and cut-out relationship among fractures, fivestages fractures can be identified: 1)fractures filled by mud and organic materials;2)fractures filled by milky calcite earlier;3)fratures filled by transparent orsemitransparent calcite later;4)structural stylolite and 5)unfilled fractures. Amongstructural fractures, mini fractures developed very well, while macro-medal fracturesdeveloped well only in local areas, such as well field of Tazhong26 along Tazhong I faultzone;well field of Tazhong9, well field of Tazhong403 to Tazhong4 and well field ofTazhong401 to Tazhong43 along Tazhong II fault zone. Most well fields are dominated byfilled structural fractures due to strongly filling action except Tazhong I fault zone wheremore than 20 percent fractures are effective, and well fields of Tazhong30, Tazhong42 andTazhong24 are the best. Tazhong II fault zone is dominated by filled fractures, percentagesof filled fractures in well fields of Tazhong9, Tazhong102 and Tazhong38 are almost 100along Tazhong II fault zone.With distribution of linear density contour, structural fractures developed well insome areas as follows: 1)fault-base zone, such as well field Tazhong19, well field ofTazhong38- Tazhong48 and well field of Zhong3 etc.;2)strike-slip part of fault, such aswell field of Tazhong102;3)tail end of dominant fault where secondary fault zonedeveloped, such as well fields of Zhong13, Zhong11, Shun2. From attitude of fracturespoint of view, fracture trend appear to be very centralized and consistent with dominantfault in the areas near dominant fault, while decentralized in the areas far from dominantfault, but appear to be consistent with secondary fault nearby.Formation of structural fracture is controlled by many factors, such as lithology,thickness of rock stratum, fault and horizon. First, structural fractures developed well inchert nodule dolomite, and the thinner the rock stratum is the more fractures occurred.Second, faults have strong control on fracture intensity. Linear density of fracture appearto be exponential decline along with augment of the distance between borehole andfault( dominant fault or secondary fault), and the decline rate is faster along secondaryfault than along dominant fault. Fracture size is also controlled by fault, i.e. macro-medialfractures exponentially declined along with the augment of the distance between boreholeand fault, while mini fractures exponentially increased, and decline rate( or increase rate )is faster along secondary fault than along dominant fault. Third, horizon also has controlon fracture intensity. Structural fractures developed well below the T74 surface, i.e. top ofYingshan formation.