Unconventional Seals for Unconventional Gas Resources: Examples from Barnett Shale and Cotton Valley Tight Sands of East Texas

摘要

Assessment of undiscovered oil and gas resources is based on geological elements and processes of a petroleum system. Application of the petroleum system in the oil industry varies largely on how the processes of hydrocarbon generation, migration, and entrapment are described. It is often depicted as a relationship between source and reservoir rocks connected by fluid paths (e.g., carrier beds, faults, etc.) through geological time. When appropriate conditions (time, temperature, and trap formation) are reached, the effort is focused on secondary migration from source to reservoir. Secondary migration efficiency is a function of the distance between source and reservoir rocks. Tertiary migration (or 'dismigration') refers to fluid movement from reservoir to reservoir and involves migration pathways (fault or sand beds and/or unconformities). There has been much research on source rock quality and its relationship to hydrocarbon potential. Much less has been documented about the rate, mechanisms, and pathways by which gases migrate through kilometer-scale sequences of fine-grained sediments. Mass balance calculations supported by laboratory experiments on good quality source rocks show that significant volumes of hydrocarbons can be generated and expelled from the source rock, but exploration results show that only a small fraction (<10%) is trapped within conventional reservoirs. Dispersion in the carrier beds (10 to 20%), retention in the source rock (30 to 40 %), 'dismigration' (10 to 20%), and biodegradation (10 to 20%) are commonly assumed to be the altering mechanisms of the bulk fluid generation. The proximity of source rock and reservoir rock becomes critical to fluid preservation and accumulation. The unconventional Barnett Shale and Cotton Valley tight sands of East Texas are no different from other petroleum systems. The Barnett Shale is a classic shale gas system that includes the elements of source, reservoir, and seal. The Cotton Valley Formation exhibits an inter-fingering shale/sand system that juxtaposes source and reservoir, offering preservation and high migration efficiency. Occurrences of sweet spots in Barnett Shale are related to the original source rock richness, maturity, and confinement of the source beds. The Fort Worth basin of East Texas is asymmetric and has a polyphased burial history. Its western part along the Washita high has undergone uplift and erosion at the Miocene. The resulting liable asphaltenes precipitation has created a permeability barrier within the shale preventing gas from escaping laterally to the west. The lower Barnett encased between the Marble Falls Limestone and the Chappel Limestone has limited gas leakage to the top and the bottom, creating an optimum seal for the New-ark Field where the highest gas production per well has been observed. Laminated carbonates and chemically induced carbonate nodule deposits in the early organic diagenesis provide vertical and lateral baffles to fluid flow thus enhance the confinement within the most productive Barnett Shale. In the Cotton Valley Formation, significant permeability reduction occurs within the interfingering shale and tight sands. The migration of oil from shale to sand has accumulated a significant volume of oil that ultimately has cracked to gas when burial reached the gas window in the Cotton Valley. This secondary cracking has resulted in high pressures extending far beyond the source rock, flushing the interstitial water to overlaying formations. Different chemical water mixes has lead to mineralization and thus diagenetic seals enhancing confinement, which has result in stair-step pressure offsets occurring independently of lithology profiles.