Conventional Traps in Unconventional Reservoir Rocks in Northern Japan

Tsuji, Takashi¹; Waseda, Amane¹; Yokoi, Satoru²
¹Technical Division, Research Center, Japan Petroleum Exploration Co., Ltd. (JAPEX), Chiba, Japan.
²Domestic Project Division, Japan Petroleum Exploration Co., Ltd. (JAPEX), Tokyo, Japan.

Siliceous rocks, mainly porcelanite and chert of the same age as the Miocene Moterey Formation in California, are distributed in the northern Japan. The rocks are not only excellent petroleum source rocks but also reservoirs and seals. Reservoir and seal studies on their burial diagenesis, chemical composition, porosity and permeability have been performed and revealed that there are at least three trap types. 1) Diagenetic transformation trap (Yurihara Type): In the diagenetic process of the transformation from opal-CT to quartz porcelanite, ultra-micro pores (pore throat radius < 0.02µm) abundantly existed in the opal-CT porcelanite disappear and well-connected micro pores (pore throat radius > 0.02μm) newly develop in the quartz porcelanite. Accordingly, the permeability of the porcelanite improves in the process. If the transformation boundary is deeper than 1000 m, the opal-CT porcelanite above the boundary is compacted enough to form a seal and the quartz porcelanite below the boundary acts as a reservoir. 2) Differential compaction trap (Toyotomi Type): The newly formed micro pores generally reduce their sizes only by mechanical compaction with burial. Clay-poor quartz porcelanite sustains good reservoir quality within one thousand meter interval below the boundary. If a clay-rich layer is interbeded in that interval, the layer is less permeable and mechanical compaction progresses selectively in the layer. As a result, the clay-rich quartz porcelanite layer plays a role of seal and the clay-poor porcelanite layer acts as a reservoir. 3) Fracture Type: The quartz porcelanite layer below the transformation boundary with the newly formed micro pores shows higher permeability than surrounding layers. If silica-rich formation water is supplied at the stage, the permeable quartz porcelanite layer changes to be a chert layer. Chert is the most brittle among siliceous rocks and capable to form fracture reservoirs. The supply mechanism of silica-rich formation water could be differential burial diagenesis of siliceous rocks due to their different clay contents and discharge of formation water due to rapid porosity decrease in the silica transformation process. Hydrocarbons reservoired in these traps show higher maturities than the reservoir rocks themselves, suggesting that the hydrocarbons have migrated from deeper source rocks same as conventional petroleum systems, although the reservoir rocks are unconventional same as shale gas/oil reservoirs.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012