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Integrated Study of Dolomitization in an Evaporite Platform Evidenced by 3-D Seismic Data and Rock Analysis: Middle Triassic Leikoupo Formation in Longgang Area, Sichuan Basin, China



Deposited in evaporative tidal-flat and lagoon setting, the Middle Triassic Leikoupo Formation (T2l) in Longgang Area, Sichuan Basin contains thick anhydrite in a large area. Recent exploration discoveries in this gas field, whose proved gas resource is more than 5.3 TCF, indicate that the distribution of reservoirs is closely related to anhydrite.

In this study, we used cores, outcrops, 3-D seismic data, and wireline logs to investigate petrophysical and seismic responses of anhydrite and its relationship with the distribution of dolomitized reservoirs. Anhydrite in the 3rd member of Leikoupo Formation (T2l3) shows high value in bulk density and gamma ray curves. A cross plot of the two log curves was drawn in 12 wells to distinguish anhydrite from other lithologies (mainly limestone and dolostone), with a coincidence rate of 90%.

On seismic profiles, anhydrite in T2l3 is characterized by strong amplitude, moderate to low frequency and moderate continuity. Seismic reflection packages tie well to lithologic profile of interbedded anhydrite, limestone, and dolostone in wells, which in turn is similar to two T2l outcrop profiles observed 150 km away from the study area. Nine Previous HitamplitudeNext Hit, Previous HitenergyTop, and frequency seismic attributes were tested on horizon slices within the reservoir interval for their ability to predict the distribution of anhydrite. Eventually, a new attribute that combines four amplitude-related attributes was found to match best with the wireline-log derived anhydrite map, with a correlation coefficient larger than 0.7.

It can be observed from the seismic attribute maps that anhydrite is widespread in the study area, with three depocenters in southwest corner, north-middle and southeast area. By comparing the distribution of T2l3 anhydrite to T2l3 and the overlying 4th member of Leikoupo Formation (T2l4) dolostone, we discovered that dolomite is closely associated with anhydrite. The dolostone is mainly distributed around anhydrite deposits. Dolomitization is interpreted to be caused by reflux of evaporated seawater in Middle Triassic. During sedimentation to shallow burial period, the formation of gypsum consumed Ca2+, leaving high value of Mg2+/Ca2+ in formation water, which benefited further dolomitization. Dolomitization mainly occurred in transition zones between evaporite ponds and palaeohighs, forming the most favorable reservoirs in the study area.