Limitation of fault-sealing and its control on hydrocarbon accumulation—An example from the Laoyemiao Oilfield of the Nanpu Sag

Petroleum Science, Nov 2008

Based on previous studies on the internal structures of fault belts, the fault belts in the Laoyemiao Oilfield of the Nanpu Sag can be divided into three units, a crushed zone, an upper induced fracture zone and a lower induced fracture zone according to the log response characteristics. The upper induced fracture zone is characterized by the development of pervasive fractures and has a poor sealing or non-sealing capability. It therefore can act as pathways for hydrocarbon migration. The lower induced fracture zone consists of fewer fractures and has limited sealing capability. The crushed zone has a good sealing capability comparable to mudstone and can thus prevent lateral migration of fluid. Through physical modeling and comparing laboratory data with calculated data of oil column heights of traps sealed by faults, it is concluded that the fault-sealing capability for oil and gas is limited. When the oil column height reaches a threshold, oil will spill over from the top of reservoir along the lower induced fracture zone under the action of buoyancy, and the size of reservoir will remain unchanged. Analysis of the formation mechanisms of the fault-sealed reservoirs in the Nanpu Sag indicated that the charging sequence of oil and gas in the reservoir was from lower formation to upper formation, with the fault playing an important role in oil and gas accumulation. The hydrocarbon potential in reverse fault-sealed traps is much better than that in the consequent fault-sealed traps. The reverse fault-sealed traps are favorable and preferred exploration targets.

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Limitation of fault-sealing and its control on hydrocarbon accumulation—An example from the Laoyemiao Oilfield of the Nanpu Sag

Pet.Sci. - SDT, μs/m 240 280 320 1.8 Compensated density, g/cm3 2 2.2 2.4 2.6 2720 2740 2760 m , h t p e D 2780 2800 2820 2720 2740 Upper induced fracture zone Crushed zone Lower induced fracture zone or only change gradually, which would result in a consistent oil-water boundary on one side of the fault like a “brush”. The following sections present the analysis about the relationship between oil column height at present and oil column boundary height sealed by the fault. The fault sealing capability could be evaluated by the displacement pressure of the fault belt. Acoustic travel time is dependent on lithology, and its response to the rock pore configuration is sensitive. Thus there is normally a good correspondence between acoustic travel time and sealing capability of faults. The displacement pressure can be calculated directly from acoustic travel time. Some researchers have made studies on this subject in the Songliao Basin, Tarim Basin and Qiongdongnan Basin. Taking the data of the Songliao Basin for example, the corresponding relationships between the displacement pressure and porosity, acoustic travel time and porosity of mudstone, silty mudstone as well as argillaceous siltstone have been established. An empirical equation between the displacement pressure and acoustic travel time for mudstone, silty mudstone as well as argillaceous siltstone has been presented (Qu et al, 2005) . According to the lithological characteristics of fault belts in the Nanpu Sag area, the empirical equation for silty mudstone is chosen to calculate the displacement pressure of the fault belt. Pc=108/(0.0745∆t 13.5745) 4.0 (1) where Pc is the displacement pressure of the fault, MPa; ∆t is the acoustic travel time of the fault belt, μs/m. Based on the sealing principle of capillary pressure, the highest oil column that the fault could seal is: where H is the highest oil column sealed by the fault, m; g is the acceleration of gravity, m/s2; w is the water density in the which is 0.8g/cm3 in0 itshitshearoeial. density in the reservoir, g/cm3, reservoir, g/cm3; The critical height of an oil column sealed by the fault can be calculated according to the above empirical equations and the statistical data are showed in Table 1. The measured data of nine reservoirs in two wells in the study area are compared with the calculated values sealed by the corresponding faults. It indicates that the deviations between oil columns in the lower reservoirs and the calculated values are small, below 7%. The good agreement of measured oil column values and calculated critical height values indicates that the reservoir is in critical saturation condition. However, the differences between the oil column heights in the upper reservoirs and the calculated values are above 8%. It indicates that they are not in the critical saturation condition. Generally speaking, the hydrocarbon sealing capability of the fault is limited. Hydrocarbons would spill over along the fault and migrate to the upper reservoirs if the oil column height in the reservoir reaches the critical value which could be sealed by the fault. The limited sealing capability of the faults can be inferred from the above discussion. The fault does not have complete sealing capability and is only effective within a limited oil column height. Smith (1966) demonstrated a theory by displacement pressure. He believed that hydrocarbon would migrate along the fault and accumulate in the lower wall of the fault if there was a difference between displacement pressures of fault belts and that of the reservoir beds (the displacement pressure of the fault belt is larger than that of the reservoir bed) until the buoyancy force caused by oil column height balanced the displacement pressure difference. capability of the lower induced fracture zone with fewer fractures and some permeability is found to be between that of mudstone and sandstone; the middle crushed zone has the best sealing and confining capability, which can prevent fluids from leaking. 2) The physical simulation, geological statistics and theoretically calculated data indicate that the fault-sealing for hydrocarbons in the Nanpu Sag is limited. Hydrocarbons will spill over along the lower induced fracture zone when the buoyancy force caused by oil column is greater than the difference between displacement pressures of the reservoir and the lower induced fracture zone. In this situation, the reservoir is in its critical saturation state, and its size reaches a maximum. The charging sequence of the faultsealed reservoirs in the lower wall of the fault is from lower formation to upper formation. The size and saturation of the upper reservoirs depends on hydrocarbons supplied. 3) Fault-sealed hydrocarbon reservoirs in the Nanpu Sag can be divided into consequent fault-sealed trap and reverse fault-sealed traps according to the dips of the fault and the formation. The former trap is sealed by the upper induced fracture zone, whereas the latter is sealed by the lower induced fracture zone. 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Zhenxue Jiang, Yuexia Dong, Hongyi Li, Luofu Liu, Guangdi Liu, Xiaoying Li. Limitation of fault-sealing and its control on hydrocarbon accumulation—An example from the Laoyemiao Oilfield of the Nanpu Sag, Petroleum Science, 2008, 295-301, DOI: 10.1007/s12182-008-0049-6