Last week I attended the AAPG convention in San Antonio. I’ve been a member of the organization for more than 30 years, and this is the first time I’ve ever had to fight for a seat at a technical session. The talks on shales, subsalt provinces and tight sands were absolutely mobbed. Typically, I look over the schedule and hop between sessions to hear selected talks. This year, if I got up and left a session my seat was immediately taken, and then I couldn’t even get into the talk I wanted to hear. The solution was to arrive early and commit to an entire session. I spent most of my time on shales, as I’m working on several shale articles at the moment. My background is in clastics, so the intricacies of shales are new to me. Knowledge about shales has exploded during the past five years. Although shales have been long recognized as prolific reservoirs in Appalachia, operators in the Barnett play in North Texas have ushered in a host of fresh ideas and technologies that seem poised to break open a multitude of new shale plays. In talk after talk, speakers presented papers that underlined the complex nature of shales. These are diverse and complicated rocks, and they hold many secrets. The term “shale:” is loosely applied to rocks that range from true shales, which by definition are fissile, through mudstones, claystones and siltstones. Indeed, it seems that the famous Barnett is not shale at all; rather, it’s siliceous mudstone. The heterogeneity in shales is astounding. They can be high- or low-pressured, clay-rich or silica-rich, and if the latter, loaded with detrital or biogenic quartz. Shales can be soft and difficult to fracture, or shatter as easily as glass. Reservoir temperatures cross the spectrum from cold to hot; thermal maturities are all over the map. Almost anything goes, and geologists are trying to figure out just what combinations of which characteristics will make a particular shale a promising producer. by Peggy Williams, Senior Exploration Editor, Oil and Gas Investor Contact me at