RESERVOIR CHARACTERIZATION

Major Chinese field evaluated using combined seismic and well logging methods

 Successful reservoir characterization in Lunnan field led to a 76% wildcat success rate and upgraded the in situ resource estimate, making it the largest carbonate field in China. 

Ronghu Zhang, Ran Zhang and Xingping Zheng and Lixin Chen, CNPC
 

In Lunnan field, an Ordovician carbonate buildup in the Tarim basin of western China, the reservoir was identified and characterized using a combination of seismic techniques and well logging methods. The seismic techniques included 3D seismic visualization and seismic attribute analyses such as coherence and root-mean-square amplitude. Well logging methods used were Formation Micro-Imaging (FMI), Electric Micro-Imaging (EMI) and Dipole Sonic Imaging (DSI). The research indicated that the Lunnan Ordovician buried hill is an oil-producing reservoir that developed under the unconformity, with a thickness of 0-150 m. The slope of the buried hill was determined to be the best location for development of the reservoir. Using the above methods, the exploration success rate was increased by 117%, and crude oil production increased to over 300 times its pre-characterization rate. The reservoir characterization also enabled a major upward re-evaluation of Lunnan field’s in situ resources.

HISTORY

Lunnan field is an Ordovician carbonate buildup that lies in the Tabei uplift of western China’s Tarim basin. The discovery well, Lunnan 1, was drilled in September 1987 and achieved production of 701.7 bpd. Fifty-three wells were drilled before 1997 for an average production rate of 20,500 bbl/yr. At that time the operator, CNPC-owned Tarim Oil Company, applied the combination of seismic techniques and well logging methods described in this article in order to better characterize the reservoir and, ultimately, to find the best well locations to increase production.

FIELD GEOLOGY

Lunnan field is bordered by the Luntai uplift to the north, the Manjiaer depression to the south, the Cao Hu depression to the east and the Halahatang depression to the west. The buried hill is a vast northeast-trending uplift that is divided by the Lunnan and Sangtamu normal grabens, Fig. 1.

The structure has experienced several phases of tectonic uplift and weathering, resulting in the Mesozoic formation being directly above the Ordovician formation, and the Carboniferous formation resting unconformably on the Lower Ordovician in many areas. Long-term exposure to surface weathering, atmospheric leaching, corrosion and tectonic rupture has also led to low matrix permeability and porosity and very well-developed fractures. The favored reservoir target of the Middle-Lower Ordovician formation is in the Yijianfang, Yingshan and Penglaiba groups with total thickness of 400-1000 m. Moving east from the buried hill body, denudation decreases, thickness increases and the formation (Upper Ordovician carbonate and mudstone) becomes newer under the unconformity.

The lithology of Middle-Lower Ordovician is micrite, micrite-grain limestone, small-grain limestone and dolomitic limestone, all of which belong to wide, quiet and clear, shallow-water carbonate deposition. Based on the lithology, fossil fragment array, sedimentary structure and rock texture, the carbonate is subdivided into interbank sea, platform inter-shoal, platform border-shoal, etc.

Reservoir geology. The Ordovician and Carboniferous strata of Lunnan field are separated by an angular unconformity. Both Silurian and Devonian strata are missing due to the lack of deposition during those periods. The Ordovician limestone has experienced a long-term atmospheric leaching and has several typical karst features.

Mudstone soil aluminum, bauxite rocks, limonite and brecciated limestone can be seen just above the unconformity. Some of the large-scale development wells have encountered caves of about 1-2 m in diameter, created by underground dissolution with collapse breccia. The caves are filled with pink sandstone, silty shale and mudstone.

Another karst feature is the high-angle cracks filled with mudstone, silt, calcite and pyrite, often with encrustation structures. Fine-grained and coarser-grained calcite are interlayered. The calcite infill has low iron content and high strontium content. Carbon and oxygen isotopic values are relatively high in the atmospheric calcite lime, with the features of epidiagenesis.

Integrated interpretation from core description, thin section observation and well log interpretation indicate that the main reservoir space is made of large cavities with diameters greater than 2 mm, fractures and cracks with widths of less than 1 mm, followed by suture associated pores and the intergranular porosity. The contribution of minor fractures to reservoir space is negligible.

The main reservoir migrating paths are systems of fractures and micro-fractures. Large cavities are generally partially filled with mudstone and siltstone breccia. The size of the cavities ranges from a few centimeters to several meters. For example, in the Well LN48, at 5,422.7 m MD in bioclastic limestone, one 30 x 40-mm cavity is filled only with fine calcite cement and mud. The cavities can be easily identified by their obvious well log characteristics, and they form an effective reservoir space, Fig. 2. Dissolved pores (with diameters of less than 2 mm) are mainly composed of small-cavity intragranular cavity, intercrystal pores and other small spaces that develop in bioclastic limestone, karst, breccias and dolomitic limestone. The fractures associated with development of karst and tectonic movement are the main effective fracture system.

The matrix porosity for Lunnan field is low. According to the analysis of 1,448 samples from 34 wells, the average matrix porosity is only 1.29% and the average permeability is 3.04 mD. The distribution of pore space is poor, and it does not contribute much to the permeability.

Four distinct types of structure can be described within the reservoir: caves structure, pores-and-cavity (1-2-cm diameter) structure, fracture-and-cavities structures and fracture reservoir. A high-quality reservoir in Lunnan field has the following features: porosity is 2.5% or greater, permeability is 5 mD or greater, capillary pressure curve threshold pressure is 2.8 MPa or less and fracture porosity is greater than 0.4%. Cavity reservoir and fractured-hole reservoir are the best two types of reservoir.

EXPLORATION TECHNOLOGY

The reservoir characterization in Lunnan field relied on a combination of seismic and well logging methods.

Imaging logging technology. Due to the distribution heterogeneity of fractures in the carbonate reservoir, it is hard to identify the reservoir and the associated reservoir parameters using conventional well logging techniques, so imaging logging was attempted.

The FMI logging works well for the identification of cavities and fractures. Large cavities appear as black areas, while small pores and cavities are characterized by “leopard-like” patterns of irregular, dark spots, Fig. 3.

In the FMI log imaging performance map, cracks appear as black rotary curves. Based on the FMI interpretation, the strike of the high-angle cracks is inclined to the northeast and northwest. Mesh-shaped cracks, conjugate joints and horizontal cracks also have been found, Fig. 4. For the Lunnan gas field, the muddy main strip, thin limestone structure and suture are very easily confused with one another based on the conventional log; FMI is a very good tool to distinguish among them.

Interpretation based on FMI can extract the area ratio of rock face to hole, the distribution of hole space, the average hole radius, and the porosities and angular orientations of cracks. By scanning the entire core diameter and reviewing the data developed from the image, the reliability of porosity and permeability information is improved.

Three-dimensional seismic technology. Three-dimensional seismic interpretation was also used to characterize the paleogeography for the Lunnan gas reservoir. The regional geologic survey shows that the fractures within the Bachu Group, which is directly above the Ordovician weathering crust, developed very well; it was deposited in a relatively stable environment. Based on 3D seismic interpretation, the surface of top Ordovician represented the geographic features for the early Hercynian period. The reorganization of karst features can predict the distribution of promising reservoirs.

Figure 5 shows a paleogeographic map of the Ordovician period in the eastern Lunnan oil field. The erosion effect is strong. The overall trend is downward from northwest to southeast. Karst slopes can be further divided into a steep karst zone and gentle karst slopes. The drainage system is very complex and controls the distribution of paleo-karst distribution. Along with the paleo-drainage system, there are the surface water and groundwater systems, the latter being controlled by the faults and rock properties.

Coherence attributes can be used to study the distribution of the fractures or the distribution of one specific lithology, and to facilitate the interpretation of the faults. There are four coherence phases. Combining the core information and well logging, white indicates a tight formation, black indicates grooves developed within the karst, gray-black indicates faults and fractures, and gray-black, star-shaped features indicate karst caves.

The reflective wave amplitude is an important kinetic parameter for seismic interpretation and lithology prediction for a given reservoir, because it can reflect the changes of lithology, reservoir properties and reservoir fluid, as well as identify the unconformity. Root Mean Square (RMS) amplitude is used to characterize the distribution of caves and fractures. When the fractures develop in the carbonate, differences can be seen due to the velocity difference among the rocks, filled fractures and water. The seismic section can be characterized by a bead-by-bead appearance, Fig. 6. The RMS amplitude attributes map 24-80 ms below the top of the Ordovician carbonate shows spot or linear features, which can be used to tell the distribution of caves and underground water.

RESERVOIR DISTRIBUTION AND PRODUCTION

Lunnan buried-hill karst reservoirs are controlled by the rock types, sedimentary environment, diagenesis, tectonic rupture, and other factors. Diagenesis and tectonic rupture play the two most important roles for reservoir development. The reservoir is mainly developed 60-150 m below the unconformity. The reservoir location can be divided into surface karst, seepage karst and underground dissolution karst zones. The main strata for the reservoir include bioclastic limestone of the Yijianfang Group and calcarenite of Yingshan Group. The limestone is well-distributed because of the development of the cave features.

For a reservoir with very well-developed caves and fractures, the RMS amplitude will be relatively high, the frequencies will be lower, the seismic cross-section will show bead-by-bead features with high Gamma Ray (GR) value, and the well log curve will be box-shaped. Large cavities appear in FMI imaging as black areas, and small pores and cavities show as irregular, leopard-like spots.

The paleogeography is very important for the development of the caves and controls the distribution of the reservoir. Usually, the main body of the buried hill is a good place to develop the reservoir, while the slope is not likely to be as productive.

With the improvement of exploration technology, the wildcat drilling success rate has increased to 76% from 35%. High-quality reservoir, in the main reservoir space of caverns and fractures, has been encountered by 27% of the wells drilled, and production has shot from an average of 20,500 bbl/yr-for the 53 wells drilled between 1990 and 1997-to 6.8 million bbl/yr for the 211 wells from 1997 to present. The reservoir characterization also allowed the re-evaluation of Lunnan field’s in situ resources from 1.9 million bbl to 7.6 billion bbl, making it the largest carbonate oil field in China.

Future research in this and similar fields should focus on techniques for characterizing fracture distribution and anisotropy in the carbonate reservoir, quantitative study of cavities and caves in the carbonate reservoir and a synthetic seismic technique with pseudo-acoustic logging curves for seismic impedance inversion.  

BIBLIOGRAPHY

Chen, J. and Z. Wang, “Tarim basin Cambrian, Ordovician reservoir characteristics and distribution of reef research,” internal report, Tarim Oil Company, CNPC, 1998.
Feng, X., Liu, X., and J. Gao, “Lunnan buried hill construction and carbonate reservoir comprehensive study,” internal report, Geophysical Institute Korla Branch Bureau, Eastern Geophysical Company, CNPC.
Read, J. F., “Carbonate platform facies,” AAPG Bulletin, 69, No. 1, 1985, pp. 1-21.
Shenghu, W., Ou, Y. and W. Tao, “Lunnan Ordovician fractured reservoir geological analysis,” Acta Petrolei Sinica, 16, No. 1, 1995, pp. 172-178.
Willson, J. L., Carbonate Facies in Geological History, Springer-Verlag, Berlin, 1975, pp. 471.
Xiulian, Z., Yinghua, W. and C. Xiao-long, “Disgenesis and porosity of the Cambrian-Ordovician carbonate shoal facies at Yangjiaping, Shimen, Hunan,” Acta Geologica Sinica, 74, No. 1, 2000, pp. 29-45.
Yang, S., Shi, H. and W. Pan, “Lunnan Ordovician insider major breakthrough in oil and gas exploration and round-east gas field discovered,”internal report, Tarim Oil Company, CNPC, 2005.

THE AUTHORS
	Ronghu Zhang is a reservoir engineer conducting research at China National Petroleum Company’s Hangzhou Institute of Geology . He received a BS degree in geology from Daqing Petroleum Institute, China, in 2000 and an MS degree in geology from China Petroleum University in 2003. Mr. Ronghu’s areas of expertise include carbonate sedimentology and reservoir characterization.
	Ran Zhang is a CNPC engineer and a PhD candidate in the Department of Geosciences at the University of Houston. He received BS and MS degrees in geology from Daqing Petroleum Institute in 2000 and 2003, respectively. Mr. Ran’s areas of expertise include sequence stratigraphy and 3D seismic interpretation.
	Xingping Zheng is a senior reservoir engineer at CNPC’s Hangzhou Institute of Geology. He received a BS degree from China Petroleum University in 1996. His main area of expertise is seismic inversion, especially in fractured carbonate reservoirs.
	Lixin Chen is a reservoir engineer for CNPC-owned Tarim Oil Company. He received a BS degree in 2004 from Chinese Petroleum University. His areas of interest include well logging interpretation for carbonate reservoirs.