Distribution Regularity of the Ordovician Reef-Bank Reservoir inside Sequence Stratigraphic Framework in Central Tarim Basin

Based on the integrated study of the outcrops, the seismic, drilling and logging data of Lianglitage formation in central Tarim Basin, the sequence stratigraphic framework of the platform margin is built, and the Lianglitage formation is divided into 3 third-order sequences. The vertical pattern and lateral distribution for carbonate reef-bank reservoir in sequence stratigraphic framework are analyzed. The reef-bank reservoir mainly developed in highstand system tract of sequence Ssq2, and distributed along the carbonate platform margin of the Middle-Late Ordovician in central Tarim Basin. The mudstone mound, organic reef, granule bank are vertically stacked. Laterally, the area of well shun3-shun4-shun2-shun6 are characterized by multiphase bank reservoir overlapping, and the area of well TZ82-TZ44 - TZ161-TZ24 - TZ26 are characterized by multiphase organic reef and bank overlapping. The reef-bank complexes in sequence Ssq2 are the main targets of exploration in central Tarim Basin.

ORGANIC REEFS OF PENNSYLVANIAN AGE IN HASKELL COUNTY, TEXAS

The reflection seismograph was used to map a prominent organic reef in the Pennsylvanian system near Stamford in central northern Texas, and the results were checked by drilling. Since seismic wave velocity in the limestone comprising the reef is the nearly twice that in the enclosing shale, distinctive reflections from beds below the base of the reef were mapped above their true position by an amount equal to almost half the reef limestone thickness. The false structure so mapped has been interpreted as showing the topography of the upper surface of the reef. As such, it is a mound 3 miles long and half as wide, with a corrected height of 1,000 feet and a central depression 300 feet deep, resembling a present‐day atoll. Numerous wells confirm the presence of this reef, but cast serious doubt upon the simple topographic interpretation of the seismic map. The atoll‐like central depression appears to be an illusion, caused by the reef having a relatively high‐velocity shell and a lower velocity center. The Stamford reef is an exceptional development along a seemingly continuous reef belt mapped similarly with refection seismograph for 25 miles. Where reef development is less extreme along this belt, and along two others of a closely related type, the reef trends show as long, as gently curving, somewhat irregular seismic “highs” with many small contours closures. Detailed seismic interpretation of these reefs, and of the structure underlying horizons which they obscure, must await improved geologic understanding of the reefs and of the subtle changes in lithology that modify seismic wave velocity. Stamford reef illustrates also what a difference there can be between seismic closure, an obvious trap by subsurface mapping, and the location of a good oil field.