--> --> Abstract: Characteristics of Karst Paleogeomorphology of Ordovician Carbonate Unconformity, Lungu Area, Tarim Basin, West China, by Jun Tian, Yanming Tong, Chunshu Luo, Yuchun Yuan, Lixin Chen, Pin Yang, Fangjian Xue, and Chunduan Zhao; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Characteristics of Karst Paleogeomorphology of Ordovician Carbonate Unconformity, Lungu Area, Tarim Basin, West China

Jun Tian1; Yanming Tong2; Chunshu Luo1; Yuchun Yuan1; Lixin Chen1; Pin Yang2; Fangjian Xue3; Chunduan Zhao2

(1) Tarim Oil Company, PetroChina, Korla, China.

(2) Data and Consulting Services, Schlumberger, Beijing, China.

(3) Data and Consulting Services, Schlumberger, Houston, TX.

The Ordovician karst carbonate reservoir is an increasingly important target for oil exploration and production in the Tarim Basin. The carbonate formation typically occurs at burial depths of 5 kilometers in the Lungu area. The paleogeomorphology of the upper part of the Ordovician carbonate directly controls the oil accumulation in the epikarstic zones. Detailed characterization of karst paleogeomorphology is one of the key steps to defining the porosity development of phreatic and vadose cave systems leading to more efficient exploration and production activities in the Lungu area.

In our study, the Top Ordovician horizon identified as the major karst unconformity was interpreted in great detail using a new technical approach. Fine reconstruction of karst paleogeomorphology was performed. On the resultant maps, the paleostream or paleovalley network was mapped in detail. The faults and/or joints in NE and NW directions formed by early Hercynian tectonic movement exhibited a reasonable relationship to the orientation of the valley. The watersheds separating drainage basins or catchment areas at different scales were identified and the structural relationships were related to early Hercynian tectonic movements. Second order paleogeomorphologic units were clearly identified, including karst highlands, karst slopes (gentle or steep) and karst basins which were apparent on either side of the largest watershed. Paleogeomorphologic units of the third order, such as peaks, remnant hills, valleys, depressions, dolines, peak clusters, karst ridges, karst lakes and their combinations were also displayed clearly. The morphologic difference of valleys was significant between the east and west Lungu area. In the west area, the paleogeomorphology is remarkable with a well-developed meandering valley oriented to the northwest. In the east area, several large zigzagged erosional valleys dominated the down-to-south drainage system. Hundreds of sinkholes, dolines or depressions were identified and almost all of them developed inside the valleys, and dry or blind valleys could be inferred.

In summary, the paleogeomorphology of Ordovician carbonate reservoir is characterized by a typical, karst topography with many youthful and mature karst features, including dolines, erosional valleys, hills and ridges. These geomorphological elements, combined with epikarst and cave reservoirs, are major controlling factors for oil accumulation and production in the Lungu area.