The Classification and Evaluation of the Ordovician Carbonate Reservoirs in Tarim Basin

Heng Zhang¹ and Zhongxian Cai^1,2
1Faculty of Earth Resources, China University of Geosciences, Wuhan, China
²Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan, China

The Ordovician carbonate reservoirs in Tarim basin have been important contributors to oil and gas production in northwestern China for more than two decades. Their architecture is complex and heterogeneous as the reservoir spaces are mainly composed of vugs, caves and fissures at different scales, which result from overlaying of different types of karstification. The classification and evaluation of reservoirs are critical for the characterization of reservoir heterogeneity.

Reservoir spaces and their combination styles are objects of the classification of reservoirs. The carbonate reservoir can be not only classified quantitatively in terms of fissure porosity and total porosity, but also classified qualitatively according to logging responses characteristics. Thus, two main categories of reservoir are recognised with the data of fields in Tarim basin: cave and non-cave reservoir. The latter is divided further into three types: (1) fissure reservoir, (2) vug reservoir, and (3) fissure-vug reservoir. The lower limit value of related parameters varies in different fields and is linked to industrial standard and the fluid properties in carbonate reservoirs. The evaluation of reservoir focuses on production in reservoir spaces. Three main categories of production level are generally stated: I category, II category and III category, through establishing the relationship of the physical property and electrical parameters bewteen the size of production.

We present two main types of fields occur around a well: electric and vadose field. The former indicates the range of radius R1, which can be investigated by logging technology and is defined as the research scope of reservoirs. The latter indicates the range of radius R2, which is defined as vadose zone. Obviously, R2 is more than R1.

The result of the classification and evaluation of reservoirs shows that the fissure or vug reservoir with extremely limited space within the range of radius R1 is also determined as I category reservoir, reflecting that the production comes from the spaces inside the vadose fields rather than electric field actually. Therefore, the category of production level is turned out to be inaccurate due to ignoring heterogeneity of carbonate reservoirs.

We propose a new category to classify the production level with two steps :(1)pick out those reservoir intervals with inconformity of electric and vadose field in the plate of fissure porosity-total porosity, (2)classify the production level of other reservoir intervals adopting the method in article.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013