The anoxic, aquatic, environment is a mass of water so depleted in oxygen that virtually all aerobic biological activity has ceased. Anoxic conditions occur where the demand for oxygen in the water column exceeds the supply. Oxygen demand relates to surface biological productivity, while oxygen supply in intermediate and deep water depends on water circulation, which is governed by global climatic patterns and the Coriolis force. Organic matter in sediments below anoxic water is commonly more abundant and more lipid-rich than under oxic water mainly because of the absence of benthonic scavenging. The specific cause for preferential lipid enrichment probably relates to the biochemistry of anaerobic bacterial activity. Recent evidence suggests that ancient organic-rich sediments containing hydrogen-rich kerogens (potential oil source beds) were deposited in similar anoxic environments. We propose the following classification for modern aquatic anoxic settings: 1. 1. Large anoxic lakes- Permanent stratification promotes development of anoxic bottom water, particularly in large, deep lakes, which are not subject to seasonal overturn, e.g., Lake Tanganyika. 2. 2. Anoxic silled basins-Landlocked silled basins with positive water balance tend to become anoxic. Typical are the Baltic and Black Seas. In arid region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-aerated, nutrient-depleted bottom waters. Anoxic conditions in silled basins on oceanic shelves also depend upon overall climatic and water circulation patterns. 3. 3. Anoxic layers caused by upwelling-Develop only when the oxygen supply in deep water cannot match demand by decaying organisms. Examples are the Benguela current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is often sufficient to match strongest demand. 4. 4. Open ocean anoxic layers-Found at intermediate depths in the northeastern Pacific and northern Indian Oceans; due to distance from deep, oxygenated polar water sources. Analogous to world-wide anoxic events at times of global climatic warm-ups and major transgression, as in late Jurassic and middle Cretaceous time. Petroleum exploration can be greatly assisted by using geochemistry to identify paleo-anoxic events in the stratigrahic record. Recognition of the proposed anoxic models in ancient sedimentary basins should help in regional mapping of oil shales and oil-source beds.