Features and origins of massive dolomite of Lower Ordovician Penglaiba Formation in the northwest Tarim Basin:Evidence from petrography and geochemistry

An integrated petrographical and geochemical study of the massive dolomite of the lower Ordovician Penglaiba Formation of the Tarim Basin,outcropping at Yonganba recognized three dolomite types:very finely to finely crystalline nonplanar-a to planar-s dolomite(D1);medium crystalline planar-s to planar-e dolomite(D2);and coarse crystalline nonplanar-a dolomite(D3).All have been affected by burial.D1 and D2 dolomites developed initially before or during shallow burial and later recrystallized,whereas D3 dolomite replaced the initial limestone entirely during burial.All three dolomites have similar geochemical features.The D2 dolomite tends to have more inter-crystalline pores(inherited from primary pores)and higher porosity due to its outstanding compaction resistance during shallow burial;whereas D3 dolomite does not retain appreciable primary pores due to strong cementation and pressure dissolution before dolomitization.This study provides a useful model for understanding the origin and porosity development of burial dolomite,in particular Paleozoic dolomite.

Development and preservation mechanism of deep and ultra-deep carbonate reservoirs

Exploration practice has proved that deep and ultra-deep reservoirs consist of mainly matrix-porous dolomite reservoirs and fractured-vuggy karst reservoirs and still will be very important targets for future exploration, in which large oil and gas fields such as Anyue, Yuanba, Halahatang, Fuman and Shunbei have been discovered. This paper systematically summarizes three theoretical and technical achievements in studying deep and ultra-deep carbonate reservoirs in the past decade.(1) The micro-zone and multi-parameter experiment analysis technology featured by determining the “age, temperature, pressure and fluid properties” of carbonate reservoirs, together with experimental simulation of cross-tectonic-period pore formation and preservation featured by the “multi-stage, continuous, visualized and online detection technology”, providing useful tools for studying the pore formation and preservation mechanism of deep and ultra-deep carbonate rocks from the perspective of“forward” and “inversion”.(2) Deep and ultra-deep matrix-porous dolostone reservoirs are still controlled by sedimentary facies,among which reef(mound) and/or beach contribute most. The reservoir space is mainly composed of sedimentary primary pores and supergene dissolution pores and fractures, though some of reservoir spaces are formed by burial dissolution and they tend to develop and may locally concentrate following the pre-existing porous zone. In other words, burial dissolution vugs are inherited rather than newly formed. Early dolomite precipitation(or dolomitization) has a high potential to preserve early pores.(3) The development and preservation mechanism of fractured-vuggy karst limestone reservoirs in deep and ultra-deep realm was analyzed. Pene-contemporaneous dissolution and interlayer and buried-hill karstification control the development of early and late supergene fractured-vuggy reservoirs. Strike-slip faults superimposed with supergene karstification lead to the development of “fence-style” faulted karst reservoirs. Dissolution simulation experiments reveal that the development of karst fracturedvuggy reservoirs is facies-controlled in certain degree, mainly developed in packstone. Rock mechanics analysis reveals that the preservation of caves is under the control of lithology, cave size, and the distance to the unconformity, and caves can be well preserved at 10,000 m. The theoretical and technical achievements provide supports for carbonate oil and gas exploration into ten thousand meters deep.