Cambrian-Ordovician Carbonate Platform And Its Restriction On Source Rocks In Tarim Basin

Based on outcrop, seismic, drilling, core and slice data, sedimentary facies in Cambrian-Ordovician in the Tarim Basin and the relationship between source rocks and platform styles are analyzed by layer interpretation, well sequences division and sedimentary facies interpretation. So it comes to the following four points:1 Cambrian - Ordovician is divided into Cambrian, the Lower Ordovician and Upper Ordovician 3 macro sequences and each macro sequence contains 3, 2 and 2super-sequences respectively. The sequence in the platform area has less symmetry of the sedimentary cycles , rising-half cycle is far less than falling- half cycle; in the slope - shelf area, the symmetry of sedimentary cycles is better, rising-half cycle is equal to falling half cycle; in the basin area, rising-half cycle is much larger than falling-half cycle.2 Cambrian generate gentle slope carbonate platforms, and Ordovician generate steep slope carbonate platforms. Cambrian platform structure style in Tabei and Gucheng area is progradation profile, the Lower Ordovician platform structure style is the retrogradation profile. Cambrian and the Lower Ordovician develop 6 and 5 reef combinations in Tabei and develop 3 and 2 in Gucheng. Cambrian and the Lower Ordovician platform structure style in Taxinan area is progradation profile, and develop 3 and 2 reef combinations respectively. Platform margin slopes in early-mid Cambrian have small changes in migration; Late Cambrian - Early Ordovician marginal slopes in the Taxinan area migrate to basin bearing. The slot in Tazhong and Tangnan slope in Tangnan area appear, the slope western of Manjiaer depression migrate to basin bearing. Four platform margin slope break presents in Late Ordovician.3 Basin facies in Cambrian - Lower Ordovician lies in the eastern part of the current Tarim basin; shelf facies lies in the transition zone between basin and slope facies; slope lies in Keping - XH2- TS1-TZ32-Gucheng4-Tangnan 1-Shache-Kandelike shelf side of the line to the zonal Spread; platform margin facies is along the above line spreading to the open platform facies side; open platform facies lies in the west of Tarim basin today; restricted platform facies lies in the central basin today; evaporative platform facies lies in the west of the basin in the lower-middle Cambrian. The basin Upper Ordovician lies in the eastern part of the current basin; shelf facies lies in the transition zone between basin facies and slope facies; slope facies is along edges of the Tabei, Tangnan area, the edge of the central uplift and the edge of Taxinan area spreading toward to shelf facies; platform margin facies is along the edge of the Tabei and Tangnan area, the edge of the central uplift and the edge of Taxinan area spreading toward to the open platform facies ; open platform facies lies in Tabei, central uplift and Tangnan area.4 The Cambrian - Ordovician develop three sets of source rocks. Source rocks in the Lower Cambrian lie in western gypsum rock area and basin-shelf- slope facies in Tabei-Tadong - Taxinan area. Source rocks in the Lower Ordovician lie in the basin-shelf-slope facies in Tadong-Gucheng-Taxinan area. Source rocks in the Upper Ordovician are located in slope in Tabei and slope in Awat-Shuntuoguole-Gucheng-Taxinan area. Source rocks, which is with gentle slope platform style in Cambrian, generate in basin, shelf and evaporate platform facies. Source rocks, which is with steep slope platform with selvedge style in lower-middle Ordovician, generate in basin, shelf and slope facies. Source rocks, which is with steep slope platform without selvedge style in upper Ordovician, generate in slope facies.