--> --> Abstract: Important Role of Volcanic Ashes in Forming Lacustrine Tight Oil Plays, by Denghua Li; #90180 (2013)

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Important Role of Volcanic Ashes in Forming Lacustrine Tight Oil Plays

Denghua Li
Research Institute of Petroleum Exploration & Development, PetroChina, Beijing, China

By means of horizontal drilling and hydraulic fracturing, many lacustrine tight oil plays have been discovered in main petroliferous basins in China for the past few years, such as the Chang 7 Formation in Ordos basin, the Lucaogou Formation in Jimusaer Sag of Junggar Basin and the Fuyu Formation in Songliao basin. Exploration cases demonstrate that organic-rich shales control tight oil plays, while cores and outcrops show that volcanic ash layers are generally observed in organic-rich shales, and more tuff layers are, the thicker the organic-rich shale is, but volcanic ash layers are rarely found in organic fair and poor shales, and it indicates that volcanic ashes probably play an important role in forming tight oil plays.

Volcanic ashes can stimulate plankton growth, and plankton blooms will form laminated organic-rich shales. After falling into lakes, volcanic ashes can rapidly release plentiful nutrient-bearing soluble salts, and it will trigger plankton blooms, thus organic-rich algae laminas will be deposited. Cores and outcrops show that tuff layers are sandwiched with laminated organic-rich shales which consist of a great many algae laminas. Experiments confirm that the plankton bloom is able to form fine-grained calcites, and some microbial dolomite microspheres, diameters 5-20nm, are discovered in core samples from the Lucaogou Formation of the Middle Permian in Junggar and Santanhu basins. Meanwhile, Cores and outcrops reveal that carbonate laminas are usually interbeded with algae ones, and they do not only form rich interlayer fractures, but also are easy to be fractured.

Volcanic ashes can grow abundant secondary porosities in the diagenetic process, and improve tight oil reservoirs property. By SEM and the electronic probe, a large number of secondary pores from volcanic ashes are observed in core samples of main lacustrine tight oil plays. Two basic pore types are identified that include intraparticle pores and interpartical ones. Intrapartical pores consist of devitrification micropores in volcanic glass shards, interlayer pores in clay minerals and intragranular dissolved pores primarily in zeolites. While interpartical pores include ones among authigenic minerals from volcanic ashes and others between volcanic ashes and organic matters or minerals.

In short, volcanic ashes are able to initiate plankton blooms, and algae blooms can form laminated organic-rich shales that control tight oil plays, also volcanic ashes can produce abundant secondary porosities that improve tight oil reservoirs property.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013